How do you get better at roleplay?

Longtime GM, rare player who has come to realize I have an embarrassing flaw. I don’t know how to roleplay spontaneity, and I don’t know how to piece puzzles together.

When I’m running games, my friends tell me I’m able to put such life into my characters. Which is easy when, you know, I control the environment and the world around them. I have a very specific lack of skill in how to build people as people. I freeze up, I don’t know how to make sudden choices.

I’m a lot better than I used to be but still, I’m not where I’m at. I just got into a VTM game so what’s your advice for roleplay?