I can think of some things which could be done ad lib without tanking the um.. situation? I have to think a little more about that statement.

When I hear this phrase, I think of so many great ad lib comedians.  Lenny Bruce, Robin Williams, Vicki Lawrence, Phillis Diller, Margaret Cho, Gilda Radner.  So many comedians giving their best with ad libs that often took a great skit over the top with a few unexpected words perfectly timed. What a gift to those of us watching these people.