Excerpt from an interview with Rob Reiner at the A.V. Club:

AVC: At the time, did you see yourself more as a performer or a writer?

RR: When you do improv, you’re everything. You’re a performer, writer, and director, because you’re moving the scene in the direction you want it to go, you’re making it up as you go, and you’re acting it. You’re all of those things, so I always viewed myself that way. And with the films I’ve done, I’ve written on them, I’ve acted in some of them. And even ones I haven’t acted in, I’ve acted them out just to be sure another actor can do them.

Read the really terrific interview here.