You can't just tell actors, especially young ones, to 'act happy' and expect them to do it. They must in some essential way be happy.
Most of us do not consciously look at movies.
It often strikes me that the actors in high school movies look too old.
We don't have a lot of class-conscious filmmaking.
One sign of a great actor is when he can be alone by himself on the screen, doing almost nothing, and producing one of a film's defining moments.
Marlon Brando is the most influential movie actor of the century.