Instead of starting with details and building out to the most important stuff, why not reverse that process? Try starting with a Core Belief, and then use that Core Belief to figure out all the other parts of your character.
Mannerism Post:
https://thejohnfox.com/2021/01/the-only-character-mannerism-list-youll-ever-need/
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Timeline
0:00 Start with Core Beliefs
1:17 What are Core Beliefs?
2:50 More Examples of Core Beliefs
3:20 Find Core Beliefs through “Unspoken Rules”
6:52 Use Core Belief to Determine Backstory
8:28 Use Core Belief to Create Actions
9:54 Use Core Belief to Create Personality
11:16 Give Your Character a Mannerism
12:53 Give Your Character a Speech Pattern
14:40 Give Your Character an Appearance