Copy, repeat and complete.
One of my favourite mantra's of Keith Johnstone is to not be creative. We are not making stuff up! Creativity is a thought process.
We want to re-act, use what is already there and read the situation to come to truly inspirational scenes. Don't use what's in your head already, use what can be found on stage by copying stuff, repeating stuff and, ultimately, completing it.
As improvisers, our true power lies in spotting all kinds of patterns and completing them as obviously as possible.