David Schwimmer is girding himself for his TV directorial debut next season when he will helm an episode of "Friends." He is "the first of the cast to do so," he points out in the next issue of InTheater magazine. "I expect (the rest of the cast)'ll be busting on me, giving me grief -- 'Who do you think you are, the director or something?'" Schwimmer has previously directed theater and the film "Since You've Been Gone." This summer he's back on the boards -- acting -- at the Williamstown (Mass.) Theater Festival in "The Glimmer Brothers," a new play by Tony-winner Warren Leight, who wrote "Side Man."