AT 3:30 on a Thursday afternoon, Conan O'Brien is at his ''Late Night'' desk at 30 Rockefeller Plaza, rehearsing the show he'll tape two hours from now. He is talking to a photograph of President Clinton on a television monitor. In a corner of the studio, seated on a wooden stool, his head on a chin rest, Robert Smigel is doing Mr. Clinton's voice. On the monitor, only his mouth is seen, moving in the space where the President's mouth should be.

''I've been better, man,'' Mr. Smigel says with a drawl, playing Mr. Clinton on vacation in Martha's Vineyard shortly after his Monica Lewinsky speech. During the sketch, Mr. Smigel, as the President, recites a limerick in which Monica rhymes with Hanukkah, uses the phrase ''like flies on Linda Tripp'' and denies having had sexual relations with Scary Spice.

This is not Mr. Smigel's real job.

''It's more like a hobby, the stuff I do with Conan,'' says Mr. Smigel, 38, who lives in Manhattan with his wife, Michelle, and their 6-month-old son, Daniel. ''It's just vaudeville.''

Mr. Smigel's ''serious'' occupation is writing cartoons for ''Saturday Night Live.'' He is the creator of the Ambiguously Gay Duo, an affectionate pair of Batman-and-Robin-style superheroes surrounded by phallic symbols, and the X-Presidents, former White House residents with superpowers. Mr. Smigel, who works with J. J. Sedelmaier, an animator, is not on the regular ''Saturday Night Live'' writing staff, although he once was.

''I'm just Cartoon Boy,'' he explains.

Comedy was not Mr. Smigel's original career plan. He was supposed to be a dentist, like his father, who developed tooth bonding. (What symmetry that the only parts of Mr. Smigel seen on television now are his lips and gleaming white teeth!)

Mr. Smigel was reared in Manhattan, first on the Upper West Side, then on the Upper East Side, by parents whose child-development philosophy, he says, was just to keep him and his sister alive.