I grew up in the theater, and you can't improvise Shakespeare and Ibsen. You have to speak the language. But obviously, in a contemporary film, there's often room for improvisation and spontaneous things that happen. As long as I know what I'm trying to achieve in the scene, and when something comes up, I know that the response is genuine, I'm comfortable. That's really how I build everything.
After you read the script, then you actually just have to be in the moment you're in, in order to make it believable. You can't give it away. You can't tip it off. For me, it's always about being truthful in the moment I'm in. Hopefully, being able to reveal what I'm feeling, you have to believe it.