First of all, relax. That's not so easy, so practice it. Reharse, rehearse, rehearse your lines until you feel completely comfortable in them, and until, as Danito pointed out, you feel the part. But don't put yourself in the straightjacket of what the director wants or even what you want the character to be like. Experiment a little. No, don't think of Sonny. An audience can always tell when an actor is imitating rather than acting.