In an electrifying series of scenes at the center of the film 'The Clouds of Sils Maria' -- when Juliette Binoche (playing a famous actress) and Kristen Stewart (playing her personal assistant) are ostensibly in character, rehearsing lines for a play -- it quickly becomes ambiguous whether the truths they express about acting and celebrity are from the fictional play, the screenplay or, most intriguingly, improvisations from their own perceptions.