I'll probably remember him most for
Mr. Mom. He was such a good bad guy. In the scene where Michael Keaton's character greets him at the door and shows him the new wing he was putting onto their house (which was BS), it was Mull who suggested the line for Keaton to say "Yeah 220, 221, whatever it takes." when he asked him about the wiring.
Keaton explaining it in an interview: “We’re doing the scene and it was okay,” Keaton
told Esquire. “And I remember saying to the prop guy, ‘Go find me a chainsaw.’ When he comes back with it, he says, ‘You wanna wear these?’ And he holds up some goggles. I go, ‘Yeah.’ You know, they make me look crazy. And when Martin shows up, I know I should look under control, I’m not sweating it. I’m a dude. So we’re standing there, Martin pulls me aside and says, ‘You know what you ought to say? When I ask about the wiring, you oughta just deadpan: ‘220, 221.’ I died. It was perfect. I may have added ‘whatever it takes.’ But it was his.”