Character diamond is a screenwriting technique that people use to "round out" their characters. It will help you create better characters.
Give the character a primary trait, and secondary trait, and they sort of conflict. Then give the guy a unshakable truth (aka permanent quality), and a quirk/flaw, and the two are also sort of... conflicting, and they conflict with each other.
For Sherlock Holmes, possibly one of the most famous fictional characters ever, primary trait is crime solver, and his secondary trait is... addict. The classic one had him on 7 percent solution, while the modern BBC one just gave him nicotine patches.
As for the unshakable truth vs quirk... Sherlock is antisocial. He solves crimes for the challenge, not because he wanted justice. But he's also extremely loyal to people he knows, like Watson. You can see they sort of oppose each other, right?
And you can see how his truth and quirk conflict/interfere with his traits. Sherlock needs to talk to people (and detect their lies) to figure out the truth.
So if you can pick the four "traits/quirks" that play into each other, that makes a great characters.
(thanks to https://wizardofads.org/use-a-screenwriters-trick-to-make-memorable-ads/ )
NOTE: Did you notice that House M.D. has basically the same character diamond as Sherlock Holmes?
This is not the only way to make a character diamond though.
Another diamond would be "persuasion strategy, true self, upbringing, facade"
For example: Indiana Jones
Persuasion strategy: hard negotiator, gets what they want
True self: how they present to close family and friends, passionate dreamer
Facade: how they present to the public, cool professional
Upbringing: scholarly family
Another diamond could be how character relates to Self / Other / Activity / Time.
* how does character relates to self?
* how does character relates to others?
* how does character relates to activity?
* How does character relates to time?
Another one would be, if dealing with fantasy:
* Racial characteristics (what race are you? what are the characteristics of that race)
* Personal/unique characteristics (what makes you unique? how are you different from others?)
* Professional characteristics (what lead you to the job you hold now?)
* Quirk (what makes you memorable? Unusual hobby or likes?)
