Improvisation, the main thing is it teaches you to be in the moment and present in the moment and be reactive and proactive for what's going on. Someone gives you something - a lot of actors are a little shut off, so they're just doing, "This is my character, these are my lines, I'm going to just send it to you then you send whatever you're sending." Improvisation teaches you to really be listening.
John C. Reilly