Neil Patrick Harris is the undisputed star of "Doogie Howser, M.D." (Wednesdays on ABC). But without another actor, it's hard to imagine its bizarre premise -- a 16-year-old doctor -- working.

That other actor is Max Casella, who plays Doogie's underachieving pal, Vinnie Delpino, a wood-be wheeler-dealer whose schemes usually have some fatal flaw.

"Well, I don't think it could work without someone like Vinnie," admits Casella, "because you need Vinnie to bring Doogie down to earth, make him look at least a little like a kid sometimes. If he's just a 16-year-old mutant-brain kid with no problems who never makes mistakes, people are going to go, 'Who wants to watch this after a long day's work?'"

Vinnie is so human himself that he would bring out the human side in anyone. "The good thing about Vinnie is that he's as true-blue as he can be," says Casella. "He's the best, loyal-est friend you could want. He goes to bat for Doogie even if Doogie yells at him or treats him badly."

And that's the reason that the relationship between two lifelong friends who logically should have grown apart years ago still works. Doogie knows that Vinnie could teach him a lot about loyalty and friendship. And around Vinnie, Doogie can be as silly and immature as he wants.

Casella is so perfect as a teenager that it comes as a shock to hear his age. "I'm 22; Vinnie's 16," he says.

But he has no difficulty playing someone that much younger. "I'm used to it," he says philosophically. "I'm always playing younger people than myself. Who would cast me as a 22-year-old?"

There's another curious thing about Vinnie. "It's supposed to take place in Brentwood, Calif.," says Casella. "I don't know where Vinnie gets his New York accent from, seeing as he grew up in Brentwood, but nobody seems to mind or bring that up."

There's a reason for that: Casella had no idea the character was supposed to be from Bentwood when he auditioned. "They like it, and it's funny -- sort of -- so they told me to keep on doing it."