It's an overly simplistic viewpoint to think of it this way, but it's a skeleton to build from:
- The writer decides who the characters are.
- The director interprets what the writer intended and figures out how to depict it.
- The actor uses various techniques to express the internal states, motivations, emotions, etc. of the character to fit the director's idea.
What I wonder is, in real movies, how many tend more towards one end or the other, really? Are actors getting way too much credit, or only a little too much?