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When I worked with Chevy Chase, Michael Ritchie would say, "Just ad lib and try to break me up. Just insult me. Anything." When we were doing his close-up, or when my back was to the camera, I would come up with jokes or quips or anything, to get a real reaction out of him. He was smart enough to know that was gold. So it was great fun working with him and Michael, and getting to see how the two worked together. I think Fletch and Clark Griswold were Chevy's two best roles. He's so incredibly talented and still vastly underused.

Source: film.avclub.com
When I worked with Chevy Chase, Michael Ritchie would say, Just ad lib and try to break me up. Just insult me. Anything. When we were doing his close-up, or when my back was to the camera, I would come up with jokes or