I had heard about about a BBC documentary on improvisation from the 1980's that featured an interview with Jerry Garcia. It took a bit of searching to track the video down, so I am linking it to it here for future reference.
The documentary is called Derek Bailey: On the Edge and Garcia is featured in part 4 of 5. I watched a little bit of some of the other segments and it looks like an interesting film as a whole.
The part with Garcia starts at about a minute and 45 seconds into the clip and runs for about 10 minutes.