Making a good (fanon) character for Until Dawn (or anything at all).
First off, huge credit for the author who made the character sheet.
Above is the link of an extremely detailed character sheet. If you want to make a simple character just for kicks, then you can probably ignore this. If you, however, wants to make a very solid character which fills in the universe of anything (including Until Dawn universe), try to fill in this monstrous list. Believe me when I say this will help you understand your character MUCH more.
In short, the character sheet will go through your basic identity, to physical appearance, personalities, past, present, his/her relation, goals, habits, traits, their self-perception and relation to other people (or social in general).
After roughly 30 minutes (spoiler alert, it took you 2+ hours to finish one character), I'm pretty sure you will feel that our character is extremely real and adaptable to your mind. Even better, you can bring your character into any roleplay possible, or writing fanfictions of your characters blending into any brave new world. Not only are your characters more defined, but the story itself they are in will be automatically enriched.
Also, sometimes a character that you prefer to roleplay with talks something about yourself, so this character sheet could be at least a tool to reflect who you might be (or, at least, might want to be).
tl;dr: This character sheet is really long but really detailed. Fill it in and you can define your character much better.