But I do think there are things that are teachable about creating character. My best general advice is always – take an acting class. Take a lot of them. Read books on acting and creating character – Michael Shurtleff’s excellent AUDITION, which is about so much more than auditioning, Stanislavski’s acting series, Michael Chekov. Learn how to develop and play characters yourself, even on a basic level, and it will translate to writing.
She has similar advice regarding antagonists.
I think this is good advice. While you don't have to have a theatre background to be a good writer, I think it can help. If you know how to get into a character's head on stage, then it will be easier to do on the page.