I think part of Izzard's success is that he doesn't take himself too seriously. He knows he's not always funny and he's not always intentionally funny. He laughs at himself as much as the audience does, which makes him feel genuine and establishes a rapport. But he keeps a fast pace so he doesn't stop and dwell on it too long. "Whoa. That worked better than I thought! Moving on..." or "So that sounded funnier in my head. Moving on!"