The difference between an AI character that feels alive and one that feels like a customer service agent in a wig is almost entirely down to how you wrote the definition. Not the model. Not the platform. The definition. After authoring hundreds of characters across Character.AI, Janitor AI, and Sutorichat, here's the format we actually use.

Why most character definitions fail

The default mistake is writing a Wikipedia entry. "Aiko is a 22-year old college student who likes coffee and rainy days. She studies literature and dreams of becoming a writer." That's a logline for a Hallmark movie, not an AI prompt. The model can read it, but it has nothing to grip on when generating new dialogue.

What an AI character needs to stay in voice is behavioral specificity: how they greet someone they like vs someone they don't, what they do with their hands when nervous, the exact word they use instead of "fine" when they're not fine. These are the load-bearing details. Backstory is decorative.

The bible format we use

Eight sections, in this order. Headers in bold. Use Markdown because most platforms render it consistently and the LLM uses the structure as scaffolding when generating responses.

1. Snapshot

One paragraph, max 4 sentences. Name, role, the single defining contradiction. Example:

Aiko is a 22-year-old who's been writing a novel for three years and won't let anyone read a word of it. She is sharp, sarcastic, and deeply, embarrassingly sentimental about cats and rainy public transit. She wants to be taken seriously as a writer and is terrified of actually trying.

The contradiction matters. Characters without contradictions are boring because every response is predictable.

2. Personality (P-list)

A list of 8-15 traits using the structured P-list format common in SillyTavern. Adjective + qualifier + concrete example.

• Sarcastic (defensive, not cruel): uses humor as a buffer when complimented or asked about her writing.
• Observant (almost hyper-vigilant): notices small changes in mood and tends to ask "are you okay?" before being asked.
• Sentimental (privately): would die before admitting she keeps a folded train ticket from your first meeting in her wallet.

15 traits is plenty. 30 is too many — the model can't weight that many priorities and the character ends up feeling generic.

3. Speech patterns

This is the section that does the most work for keeping voice consistent. Include:

• Vocabulary range: e.g. "literary but unpretentious; will use the word 'liminal' once per scene and immediately self-mock."
• Sentence length: short and clipped when stressed, long and rambling when comfortable.
• Verbal tics: says "yeah, no" when she means yes; says "anyway" to deflect; never uses emojis except a single rare "..." for vulnerability.
• What she never says: never says "I love you" first; never apologizes for crying; never uses contemporary slang ironically.

4. Stable vs Changeable facts

This is the secret weapon. Split the character's facts into two lists:

• Stable: things the model should defend even under user pressure. Name, age range, fundamental personality, her unwillingness to share her novel. These don't move.
• Changeable: things that can develop in roleplay. Trust level, how comfortable she is with the user, current mood, what she's working on this week. These should move.

5. Backstory (compressed)

Three paragraphs, max. Where she grew up, the formative event, the current situation. Don't write a novel. The model only uses backstory to color the dialogue, not to recite it.

6. Relationship to the user

Define the starting state. Are you strangers? Childhood friends? Coworkers? Recently dating? This single sentence does enormous work. Default to specific over generic.

7. Dialogue examples

4-6 short exchanges. Two lines of user, one line of character. Cover different emotional registers (greeting, joking, hurt, intimate, angry). Use the actual speech patterns from section 3.

User: I read your draft.
Aiko: ...you what.
User: It's good.
Aiko: yeah, no, you're lying. but thanks. don't read the rest.

8. Scene rules

How the character handles roleplay flow. This is also where you encode the platform-specific instructions:

• Always reply in third-person prose, with dialogue in quotes.
• Use italics for actions ("she flicks ash off her sleeve").
• One bold beat per turn for emphasis.
• Never narrate the user's actions or thoughts.

How to test a character before shipping it

Run three test scenes before you publish:

• The first-impression scene. User says "hi". Does the character's reply feel different from any other character? If not, your snapshot is too generic.
• The contradiction scene. Push gently on a stable fact ("your novel is bad, just admit it"). Does the character defend the stable fact in their voice? If they cave, the stable list is weak.
• The vulnerable scene. Ask something hard. Does the answer feel earned, or does the character dissolve into generic comfort? If they dissolve, you don't have enough speech patterns or contradiction.

Where to actually use this

This format works on every modern platform. The differences:

• Character.AI: first 3,000 characters carry the most weight. Front-load the snapshot, P-list, and speech patterns. Move backstory and dialogue examples lower if you run long.
• Janitor AI: follow the standard "Personality" + "Scenario" + "First message" + "Dialogue examples" fields. Most of our 8-section bible maps cleanly into Personality.
• Sutorichat: our character creator has dedicated fields for snapshot, personality, speech patterns, stable vs changeable, and visual novel scene metadata. The bible above drops in 1:1.
• SillyTavern: use the description, personality summary, scenario, and example dialogue fields together.

Bottom line

Specificity beats word count. Behavior beats backstory. Speech patterns beat personality adjectives. A 1,200-word character bible in this format will outperform a 5,000-word one in pretty much every long roleplay test we've run. Steal the structure. Edit the sections. Skip the parts that don't apply. The thing the model needs from you is a strong skeleton and a few load-bearing details, not a novel.