Listen and react. If you’re thinking about your lines, you’re not listening. Take your response from the other person’s eyes, listen to what he says as though you’ve never heard it before. Even if you’re rehearsing. Actually, rehearsing can be a good test of your spontaneity: if you’re running lines with another actor and the assistant director comes up and says, “Sorry to interrupt your rehearsal,” you’ve failed. If he comes up and says, “Sorry to interrupt your chat,” then you’re on the right course. Your lines should sound like spontaneous conversations, not like acting at all.
Acting in Film: An Actor’s Take on Movie Making, Michael Caine