Headcanon Ideas That Survive an AU: A Three-Rule Consistency Checklist

Most AU advice starts with the setting. This starts one step earlier, at the sorting: given a pile of headcanon ideas for one character, which of them are actually about the character, and which are about the world you happened to write them in first? Get that step wrong and no amount of consistent naming or careful continuity will hold the voice together.

Three rules sit between the setting and the character. Only ideas that pass all three are still headcanons once the world around them has been replaced; everything else was scenery.

An AU changes the world, not the person

An alternate universe changes the answer to "what is this world like". It does not change the answer to "what does this person do when they are scared, tired, or needed". Those are two different kinds of sentence, and quietly mixing them is where consistency goes wrong. The first kind has to be rewritten for every setting. The second kind should not have to be rewritten at all, and if it does, it was never about the person.

The useful part is that most headcanon lines are already the second kind. In this site's own bank of 180 hand-written lines, 165 of 180 name nothing that belongs to one specific setting — no job title, no organisation, no invented world, no technology. 15 mention a modern institution: a job, a group chat, a school award, a bus route, a charger. Zero mention anything fantasy or science fiction — no kingdom, no throne, no starship, no spell. The word lists and the counting rule are given further down the page, so you can check every number against the source file yourself.

That matters because it moves the blame. If a character stops being recognisable after you move them, the usual suspect is not the headcanon you picked. It is the world you wrapped around it afterwards, plus the decision to keep describing that world in your lines instead of describing the person.

The three rules

From your pile, pick three sentences and freeze them. Everything else is negotiable; these three are not.

• Rule 1 — What they put first. Under pressure, what does this person reach for before they reach for themselves? Not a virtue and not a flaw: an order of operations. Someone who makes the room calm before they make their own point. Someone who finishes the job before they take the credit. Written as behaviour, this sentence survives a change of century, genre, or species.
• Rule 2 — The line they never cross. One thing they will not do even when it would work, and even when nobody is watching. This is the rule that stops a character from becoming a different person the moment the new setting offers them a new temptation. It is usually phrased as a refusal, or as a secret they will not hand over.
• Rule 3 — How they treat someone with less power than them. Not their friends and not their equals: the new arrival, the junior, the person who cannot repay them. This rule travels further than any other, because every setting — coffee shop, starship, village — contains someone with less power, and the way they are treated belongs to the character, not to the world.

Then test each rule by asking the same question twice, once in the original setting and once in the AU. If the two answers differ, the rule was not frozen yet. If they match, you have something you can build a whole story on without checking your notes every scene.

What the three rules look like on real lines

These five lines come from a fixed roll of this site's bank — character name Marlow, all five categories, all six tones, five lines, first roll for that name. The reproduction steps are at the end of the page.

• Marlow practises kind sentences in advance so there is one ready when it is needed.
• Marlow sits with people through hard things and never mentions it afterwards.
• Marlow treats every minor inconvenience like a personal rival and keeps score.
• Marlow once joined a club for the snacks and ended up running it for two years.
• Marlow has one specific mug that only they may use, and everyone has accepted this.

Read as a set, the first two lines hand you two of the three rules for free. The first is Rule 1 stated as preparation: the thing this person reaches for first is the right sentence, and they would rather have it ready than improvise. The second is Rule 3 stated as an absence: they sit with people through the worst of it and then decline to be thanked. Neither sentence contains a single word that belongs to a world, which is why both would survive being moved into a spaceship or a village without an edit.

The last three lines are quirks, and that is worth noticing: not every line in a pile is a rule. Quirks describe how a person is, not what they will not do, so they are cheap to keep and easy to lose. If a setting change forces you to cut something, cut from here first.

Rule 2 is missing from this batch, which is normal — refusals are rarer than habits. A second roll on the same name produces one: Marlow keeps an emergency snack in their bag and has a prepared story about why it is there. The behaviour underneath the snack is the rule: this person will not admit, out loud, that they need looking after. Put that sentence in the freezer with the other two and you have all three rules from a single name and two rolls of the generator.

Worked example: five headcanon ideas, three settings

Here is the next roll for the same name, five lines again. This time move each line into three different worlds and watch what actually has to change.

• Marlow keeps the key to a place that no longer exists.
• Marlow is fluent in the kind of tired that sleep does not fix.
• Marlow keeps an emergency snack in their bag and has a prepared story about why it is there.
• Marlow owns three copies of the same book and none of them is the one they are looking for.
• Marlow says I'm fine to the one person who can tell, because that person is the only one it would hurt.

Now the three settings:

• A coffee shop, present day. Nothing to change. Every noun in all five lines already exists here.
• A crew on a long-haul ship. The key becomes a key to a cabin that was reassigned a year ago; the snack becomes a ration bar kept out of the stores; the three copies of the same book become three copies of the same file, none of them the current version. The behaviour is untouched in all three cases — only the carrier of the behaviour changes.
• A village a long time before plumbing. The key becomes a key to a house that has been pulled down; the snack becomes dried food wrapped in cloth; the book could stay exactly as it is, because books are older than plumbing. Again, the sentences that had to change were the ones carrying an object, and the sentences about the person did not move at all.

Count what happened: five lines, three settings, fifteen versions, and not one line had to be thrown away. Three of the five needed a noun swapped. The other two — the tired that sleep does not fix, and the "I'm fine" said only to the person it would hurt — travelled without a single edit, in any direction. Those two are the ones worth writing down permanently; they are the reason the character is still the character in the third setting.

Now do the same for a line that fails. "Works the night shift at the docks" is a perfectly good sentence, but it is a sentence about the docks. In a village it needs the docks, in a ship it needs a shift rota, and if you keep it anyway you will spend three paragraphs explaining an institution the reader does not care about. Cross it out and spend the space on Rule 1 instead — this is the one category of line where deleting is the correct move rather than a loss.

The two-way test

Before you keep a line, run it backwards. Take the sentence as it now reads in the AU and drop it into the original setting. If it still describes something this character would do, it is a headcanon and it belongs on the list. If it collapses the moment the world changes back, it was set dressing wearing the character's name.

This test catches the most common AU failure by design: a line that is emotionally true in the new setting but has no counterpart in the old one, which usually means it was written about the premise instead of the person. It also catches the opposite mistake — a line so generic that it survives by saying nothing. If a sentence can be attached to literally any character in any setting, it fails for a different reason: it is not an idea, it is a template. Both failures look like success on a first read, which is why the test has to be run deliberately, one line at a time.

It takes about a minute per line and involves no rewriting, which makes it the cheapest consistency check available once the three rules are frozen.

The checklist

Copy this and work through it in order. It takes one sitting per character, and it holds for a short AU piece and a long rewrite alike.

• Step 1 — Roll the pile in the original setting first. Not in the AU. You want the character before the world gets its hands on them.
• Step 2 — Mark the three rules. Underline what they put first, the line they never cross, and how they treat someone with less power. Write all three as behaviour, not as adjectives.
• Step 3 — Circle every noun that belongs to a world. Jobs, places, tools, institutions, era words. Those are the only parts allowed to change.
• Step 4 — Transplant the nouns, leave the verbs. Swap each circled word for its equivalent in the new setting. If a noun has no equivalent, the line is not transplantable — delete it rather than explain it.
• Step 5 — Run the two-way test. Drop each surviving line back into the original setting. Keep the ones that still read as true, and delete the rest.
• Step 6 — Read the three rules aloud. If the three frozen sentences have not changed, the character crossed over intact. If any of them moved, you have written a variant rather than the same person — which is allowed, but it is a different project and should be labelled as one.

Print it, or keep it open beside the draft. The point is that it is short enough to actually run.

The fill-in template

Six blanks. Fill them once per setting; if the last two match across settings, you are done.

• Setting: ______
• What they put first: ______
• The line they never cross: ______
• How they treat someone with less power: ______
• The behaviour that must read the same in every version: ______
• One thing I am willing to lose if the setting demands it: ______

The fifth blank is the one to protect. The sixth is the one that keeps you from breaking the piece by insisting that every detail travels; some things genuinely are scenery, and letting them go is part of the job.

Where these numbers come from

Every count on this page was taken from this site's own source file rather than estimated, and the rule is short enough to repeat in full.

The bank. The generator on the front page reads a single hand-written object holding 180 lines, arranged as 5 categories (Personality, Backstory, Relationships, Quirks, Secrets) times 6 tones times exactly 6 lines per cell. All 180 lines open with the character name, and each line uses the name exactly once. Line length runs from 46 to 143 characters, with a median of 81.

The counting rule. Lower-case every line, remove the name placeholder, drop function words, count each word once per line, then sort by how many of the 180 lines a word appears in. Applied that way, the three most widely shared words in the bank are friend, learned and room, each appearing in 9 of the 180 lines. The highest-ranked concrete nouns are no more specific than group, at 4 lines, and book, phone, food and home, at 3 lines each. Nothing that belongs to a specific world appears anywhere near the top of that list, and that distribution is the reason the three rules are cheap to apply: the bank is written mostly at the level of behaviour, so a setting change has very little to grab onto.

The three lists. The world-dependency figures come from matching each of the 180 lines against three explicit word lists. The modern-institution list has 74 entries — work, job, office, shift, meeting, deadline, contract, money, bill, client, manager, program, software, chat, phone, email, online, licence, uniform, exam, degree, hospital, doctor, court, police, train, bus, car, school, class, church, army, salary, career, employer, colleague, staff, business, market, bank, tax, insurance, account, camera, and similar words in the same families. The fantasy-and-sci-fi list has 56 entries — kingdom, throne, king, queen, lord, knight, sword, shield, dragon, magic, spell, wizard, tavern, inn, guild, village, castle, starship, planet, station, colony, alien, robot, laser, hologram, clone, empire, vampire, demon, temple, prophecy, quest, crown, battle, and similar. The everyday-object list has 52 entries — room, door, shelf, box, cup, coffee, tea, book, note, phone, chair, window, bag, key, letter, photo, jacket, umbrella, table, kitchen, garden, street, menu, dog, wall, floor, plant, notebook, pen, and similar. A line is counted once per list no matter how many entries it matches: 15 lines hit the first list, 0 hit the second, 45 hit the third.

Why the lists matter to the conclusion. The judgement call here is the word lists, not the arithmetic. Disagree with a word and change it — the source file is static, so the count is reproducible either way. What no reasonable list will produce is a run of fantasy or science-fiction lines, because there are none in the bank to find.

Reproducing the rolls

A roll is seeded from five inputs: the character name, the selected tones, the selected categories, the line count, and a seed counter. Changing the name, the tones, the categories or the count resets that counter to 1, and each press of the generate button moves it on by one. That is why the first roll after a change is the one worth quoting, and later rolls should be quoted with the press count that produced them.

To get the ten lines quoted above: open the generator, type Marlow into the name field, leave all six tones and all five categories switched on, set the count to 5, and press generate once. The five lines under the heading about the three rules are what the count change produces; the five in the worked example are what the following press produces. The archetype label shown above the cards comes from the same seed, so it can be used as a second check that you are looking at the same roll.

Two related notes, both checked against this site rather than taken from elsewhere. First, rolls do repeat once you have drawn enough of them; the bank holds 180 lines, and 180 is not a large number. That behaviour is described in why headcanon generators repeat. Second, every line in the bank is a single sentence about one person, so a line that appears to describe two people is really describing one person's way of treating the other. For the version of that problem where a whole group needs its own dynamics, see party dynamics.

Quick answers

• Does an AU need its own headcanons? Some of them, yes — an AU invents new conditions, and conditions generate their own consequences. But the three frozen rules are not among them. Write the new consequences around a fixed core instead of in place of it.
• What if the AU changes the character's age or species? Then Rules 1 and 3 still hold and Rule 2 needs a review, because a different body and a different lifespan change what a person is capable of doing. Freeze the rules that survive, and note the one that does not rather than silently rewriting it.
• How many ideas should I freeze? Three. Two is a mood rather than a person, four is a summary nobody can hold in their head while writing, and past four you will contradict yourself somewhere around scene three.
• Can I keep the same three in a sequel AU? Yes, and you should — that is what makes the second work read as the same character. Just remember that a line which passes the two-way test travels, while a line that only works because the new AU offers it a new temptation does not.

None of this needs notes, software, or a paid tool. It needs one pass through a pile of lines with three sentences held still, and a willingness to delete the pretty lines that turn out to be scenery. If you are still deciding what to put in the pile, the category and tone matrix covers which kinds of line to roll for, and turning a single relationship line into a scene covers what to do with the ones that survive. All guides are listed in one place.