Hey, it's me. said:
Both. You can't differentiate between the two. Plus a decent script and and a director who knows how to tell a story. (Irwin Kershner springs to mind)
True, but a talented and charismatic actor can make a badly written character interesting (Christopher Lee), but a lousy performance can ruin what should be a great character.
Although, given this is a "Star Wars" forum, to be fair, I don't think any actor could have saved Anakin in the PT.