Reddit mentions of An Actor's Work: A Student's Diary

You're right, that is a very, very broad question. It's similar to asking "what's the full range of what you would learn at a conservatory/MFA program?" or something similar. You could check out Stanislavski's work. It's comprehensive. There's a new consolidated translation of what used to be two books, now called An Actor's Work. That would be a great place to start.

The answer to your question will vary from actor to actor and from role to role. Generally there's a lot of script analysis, learning as much as you can about your character from the script itself. What he does, what he says, what others say about him. Making decisions about what he's trying to accomplish, what he wants, what's driving him, what's holding him back. If it's a historical character or one who can be compared to a historical character, then you can do some research there. You can explore how they may move and speak differently than you normally move and speak, and incorporate that. It may not be different, but I think it's always worth considering.

There's obviously much more to it than that, exploring in rehearsal (a given for stage, not necessarily for camera work) and dealing honestly with your partners and letting them affect you and allowing your impulses to come out freely.

Good advice, but those aren't the best translations of Stanislavski. This more modern version is supposedly much truer to the original. There's also a reading list in the sidebar.