EN CARDE, NILES OF ALL THE SWEEPS STUNTS IN MAY, perhaps none is more startling than the next episode of NBC's Frasier (Tues., May 2, 9 p.m. ET), which finds the exquisitely repressed psychiatrist Niles (David Hyde Pierce) engaging in a swashbuckling duel with a Bavarian fencing champion for the affection of Niles's wife, Maris. It is as acrobatic as any of the fight scenes in Rob Roy. "Took me about a week to recover," says Pierce, who is now rehearsing the Oliver Stone film Nixon (he plays John Dean). Pierce credits his proficiency with a sword to his classical theater background. "I did a couple of Cyranos in which I worked with B.H. Barry, who is a terrific fight director," says the actor. "And I was Laertes to Kevin Kline's Hamlet. At the same time I understudied Kevin as Hamlet. So I had to learn both sides of the sword fight for that. You basically have to learn to fight with yourself." Sounds like just the sort of inner conflict Niles loves to analyze.