Our feeling is that the most important thing on a set is that actors have enough confidence to try different things. If there's stress or tension, they won't go out on a limb because they won't want to embarrass themselves if they don't feel completely comfortable.
The problem that I have is with the music business. For some reason it seems almost impossible to get anything, any music, released which includes improvisation or soloing.
I spent a lot of years just learning my craft and falling down in front of the camera.