You are not going into the next scene of your story knowing beforehand what to do or what to say.
There is no script.
The answer then is to do improv.
The thing is, if we are spending our time demanding our boss for a script–a simple set of instructions to follow, or asking if this will be on the test, we are squashing a world of wonder and possibility.
No one is good at improv at first. Improv takes time to learn. We all stumble learning how to throw the ball in the air (and catching it).
Remember: We are all making things up as we go but all behavior makes sense from the actor’s perspective or else the actor wouldn’t do it.