Exclusive: Jon Hamm has proved himself so adept at comedy on “30 Rock” and “Saturday Night Live” that it’s about to pay off in movies. I’m told that Hamm is about to sign on to play Larry David’s nemesis in the still being worked out improv film David is set to shoot this fall. The film, without a title, will star David as a Larry David like character–but with a different name. He’ll get into the same kind of situations as Larry David on “Curb Your Enthusiasm.” Greg Mottola is directing, and the “script-ment”–a script-slash-treatment–is being worked up by David and his “Curb” and “Seinfeld” writers. Fox Searchlight will distribute. All of this is still in the planning stages, which means it will be some time before we see another season of “Curb” on HBO. But Don Draper as Larry’s sparring partner could be very funny. Hamm’s recent turn on the live version of “30 Rock” showed that he’s capable of a lot more than smoking and brooding.