Adapt the role to the actor, not vice versa.
In an interview on the Coup de Grâce DVD, German director Volker Schöndorff (Academy Award-winner for Best Foreign Film The Tin Drum) gives his key insight into how to fit performances to actors. Like many directors, he believes casting is key. But he takes the principle even further:
I find that once casting is done and you have all the actors, the actor becomes more important than the role. You have to adapt the role to the actor, and not vice versa, once the casting is done. Because every film is a documentary. Or at least a document about that actor. What the camera sees is not the acting. That's the performance; that's the construction. But the camera also sees the man or the woman.
I think for most actors this is good advice. But many of the top actors are such incredible chameleons (Meryl Streep, Daniel Day-Lewis), that it makes more sense to let them fit themselves into the movie.
What do you think? Should a director bend the script and other supporting elements (costume, lighting, camera movement...) to suit the natural demeanor of an actor or actress? Or should everything serve the script, even the actors?
