"Is there a doctor in the house?"

Well, if that house is Studio 54, the answer — through mid-March, at least — is "Yes!" This is where "Doogie Howser, M.D." is now making house calls eight times a week — a long, long way from his sitcom home where he did rounds for four of his more formative teen years (1989-1993) as precocious-medico-in-residence at L.A.'s Eastman Medical Center.

Now, he calls the shots at a seedy Berlin nightspot in 1929-30 when swastikas are starting to sprout like wildflowers. As the Emcee in Cabaret, he hovers darkly over this doomed demimonde — a mincing menace who jumps into the forced fun with both cleats and socks across some swell Kander & Ebb.

If you think The Kit Kat Klub is a wildly unlikely Doogie house, you are not alone. "It's a role I never anticipated performing," admits Neil Patrick Harris, the long-gone Doogie whose boyish good looks would more readily qualify him to play Cabaret's incorrigibly corruptible Clifford Bradshaw. "If the movie version of Cabaret came up again, I'd never get the opportunity to do the Emcee. I'd be thought of as a Cliff. That's why I'm so grateful to the Roundabout for giving me the chance to do something more extreme."

He got this shot because of a good working relationship with Roundabout Theatre — make that "good workshopping relationship" (he has done two workshops of Assassins, which Roundabout will do next year). "We've been trying to find a time for me to do the Emcee for a couple of years," says Harris, "and it was never the right time. Now, it works out well. For one thing, I'm more calm in my own skin, which I think is necessary for that role. It's so physically specific, such a full-bodied ride for the actor playing the Emcee. Your physicality defines you throughout. Even when you're sitting and observing, which happens a lot in the show, you're still so present. It's not a close-up. It's a wide shot." Harris was too young to see Joel Grey's Tony-winning Emcee, but he did catch his Oscar-winning one, and he's a huge fan of Alan Cumming's Tony-winning reprise. "I was really impressed at how Sam Mendes restaged this production, turned it inside out by making the environment part of the show as opposed to sitting in a theatre watching a cabaret and having the scenes be separate from the show. Now, to sit in a cabaret itself and have scenes be sketches within this cabaret show includes the audience in a more profound way — so much so that, by the end of the show when it becomes a much darker, more twisted reality, they participate because they have become so involved theatrically."

His one fear is that people might see this assignment as stunt casting. Comparisons are odious and, in Harris's case, uncalled for. "Doogie Howser, M.D." was merely a calling card to fame, his first ticket to ride — and it didn't take into account his musical gifts.

"I come from a musical family. My dad played a guitar, and my mom the flute. I grew up singing in choirs at church and in elementary school. I was always musically inclined."

But that didn't get him Job One. Mark Medoff, author of 1980's Tony-winning Children of a Lesser God, spotted Harris at a drama camp in New Mexico and thought him perfect for a part in a Whoopi Goldberg picture he'd written ("Clara's Heart"). The boy was put on tape; that got him an agent, who got him the film and has kept him working since age 15.

"Doogie Howser, M.D." was a decidedly mixed blessing. It made him a household face, but it also made it difficult for people to see him as anything else — a short-sightedness Cabaret should thoroughly demolish. "I think people will see I've aged a bit, a little more out of the box," he understates. "What's amazing is that the bulk of my television work ended in 1993. It's so shocking to me that a decade has gone by, and still it's memorable in people's minds. There were lots of syndication reruns so it was still on, but now even that's gone. I'm amazed at the impact that four years of a show can have."

The pigeonholing became apparent when Harris finally played the musical card big-time and marked it well with raves as Mark in the L.A. production of Rent. "I felt they thought it was stunt casting. They were surprised I could actually do it. It's nice that I took that challenge, but I wasn't performing Rent to prove to them that I was good. I was performing to show the audience what Jonathan Larson had written. I was a big proponent of what that show said, so it wasn't a selfish performance. It was a very decisively dramatic performance. I wanted them to get the drama and story of the show. I wasn't there as a rock star. I was there as a storyteller."

The role and the raves won him a supporting berth in an L.A. edition of Sweeney Todd with Kelsey Grammer and Christine Baranski — Tobias Ragg ("Not While I'm Around") — and an impressed Stephen Sondheim brought him East to do it again for the Lincoln Center version with George Hearn and Patti LuPone, a production that was eventually taped by PBS in San Francisco. The Sondheim association has also put him in good stead to play not one but two roles in the upcoming Assassins. "Apparently, Steve originally wanted the balladeer to become Lee Harvey Oswald at the end and to be played by one actor, but they couldn't really quite figure out how to make that work. With this new production, [director] Joe Mantello wants to re-explore that and have the constantly cheerful nature of the balladeer turn into the darkness that becomes Oswald."

Because Assassins has yet to be on Broadway (and, therefore, eligible for Tonys), this will be his first non-Tony-winning show. Harris, however, looks at it differently: "I only do plays that have one word in the title — Rent, Proof, Cabaret, Assassins. Pretty soon, we'll be getting to the two-word shows." Either way, the "doctor" is in — for the long haul.