Most of the movie stars you see in Pittsburgh are on the screen, not at the concession stand buying popcorn right next to you. But the few people who went to the late showing Monday night of "U.S. Marshals" at the Showcase North got more than Tommy Lee Jones and Wesley Snipes. Matt Damon, Ben Affleck, and Kevin Smith dropped in together, taking a break from rehearsals of "Dogma", which they begin filming here next week. Don't look for them at Showcase North next Monday night. Damon and Affleck will be in L.A. for the Academy Awards, where the pair is nominated for best original screenplay for "Good Will Hunting" and Damon is up for best actor.