What do you do to get into a premade character's mindset?

I'm going to be playing my first game where the character and their background is written by the game runners. It's a rather different character than what I normally do, but one I'm excited to take on. I've been trying to get into his mindset by watching movies with similar protagonists and rehearsing mannerisms that would suit him in the mirror.

I'm wondering if anyone else has tips or tricks for entering a character's headspace and preparing to play them. What do you do to get into the mindset of a character that's very different from you and what you normally play?