From Radiolab, Paul Ekman describes his definition of lying:
A lie is a deliberate choice to mislead a target without any notification.
According to that definition an actor is not a liar, although… I saw a good actor last night in a play and I was for a time misled. I even had tears because he had misled me. But I was notified.
My wife taught me what I’m supposed to say when she comes in with a new dress – I’m not supposed to say, “Gee, that’s not a flattering cut,” or, “the color is wrong,” or, “that’s for someone twenty years younger,” all of which might be true. I’m supposed to say, “Smashing!” So I’ve agreed to those rules, and since we’ve agreed about that I’m not lying.
It’s like when people say “how ya doin’?” in the hallway. The correct answer is “OK” because no one wants to hear about the mysterious pain in your knee, or how your kid keeps setting fire to his toys. Or about the voices in your head, for that matter.