As a performer, it's easy to practice certain looks, responses, reactions, and characters in which one excels. However, it's also easy to get into a rut if those practiced actions become canned. How do you keep them fresh and new?
It may sound like a contradiction, but you achieve spontaneity on the set through preparation of the dialogue at home. As you prepare, find ways of making your responses appear newly minted. In life, we often pick up the thought that provokes our next remark halfway through someone else's speech. Thoughts don't leap to the mouth automatically.We don't interrupt at every occasion when a thought formulates itself; or, if we do, we don't have many friends. Similarly, in a film script, your internal thought processes might well start articulating themselves long before you get the chance to speak. The script sometimes directs you to interrupt, but if it doesn't, your thought may start well before you get the chance to respond. There may be a key word that triggers you during the sentence the other actor is saying. So pick up on that; form your thought and be ready to speak. For example:
Other Actor: "I've got to get a bus to Clapham -- I'm already late for my date.
You: You won't get far. There's a bus strike.
The other actor doesn't stop talking after he says "bus," so you can't get in and say your line at the actual moment of thought recognition. But when you hear the key word "ubs," from that point on you knowo what you're going to say directly after he stops. You can show this by your reaction. And that bit of acting can only come from serious listening.
Knowing your next thought trigger will help you stay in character and in the moment. And as the audience, we can feel you anticipating what you want to say, which adds intensity to our expectation. What script have you had recently that this tactic could be helpful in?