Those zany doctors and nurses on "Scrubs" had better shape up: There's a new shrink in-house.

Heather Graham ("Boogie Nights") joins the critically acclaimed NBC sitcom for an eight-episode story arc as Dr. Molly Clark, starting with the season premiere on Tuesday at 9:25 p.m., a bit earlier than usual.

As viewers quickly discover, Molly is beautiful, sunny and smart -- at least in matters relating to her field of expertise.

She's also a bit spacey, especially when it comes to remembering the names of colleagues. Oh, and she sings to her food.

"I love that she's so many things at once," Graham said, after recently wrapping a day of filming. "That's how real people are. I mean, when it comes to something I know about, I'm actually pretty ... smart, but in other areas it can be, 'Uh, where am I?' "

"Scrubs" creator and executive producer Bill Lawrence, who also directs Graham's first episode, said, "The cool thing is that Heather's a lot different as an actress from the other people on the show. . "After four years of any TV show, everybody has this very established and succinct rhythm. Our show is very fast-paced, and Heather comes on and is different because she hasn't been in this world (either as an actress or a character). It's fiction and reality overlapping a bit. The character of Molly is much nicer and sweeter and less edgy and cynical than the other people there," he said.

Lawrence maintains a very firm "no jerks" attitude on the "Scrubs" set, which means that most guest stars previously have been Lawrence's personal friends, such as Brendan Fraser or his old "Spin City" colleagues Michael J. Fox, Heather Locklear and Barry Bostwick. "And if I don't get Michael Boatman on this year, he will kill me," Lawrence added.

Apart from that, it's pretty much by invitation only, although Lawrence cheerfully admits that isn't always as hard as it may sound.

"Someone will come up and tell me how much they like 'Scrubs,' and we're easily flattered and star-struck, so that usually leads to 'Well, hey, want to come on the show?' " he said.

For Graham, her invitation came while working on the film "Cake" with Sarah Chalke, who plays Dr. Elliot Reid on "Scrubs." Graham never had seen the NBC hospital sitcom until she heard others on the film talking about how good the TV show was. She quickly became a fan.

"At first, I was a little worried about coming into a show where the ensemble is this tight," Graham said, "but then Bill Lawrence told me, 'Molly is going to become the close friend and mentor of Sarah's character,' and I immediately felt more comfortable. And, I know it sounds a little corny, but these truly are the nicest people I've ever worked with."

To prep for her new gig as a psychiatrist, Graham said she read a few books Lawrence had recommended, chatted with the "Scrubs" medical consultant, and talked to psychiatrists about their work.

"And, of course, I had my own 10 years of therapy to draw on," she said with a laugh.

Graham, who guest-starred on "Growing Pains" as a teenager, said she had forgotten how fast-paced sitcoms can be, but she loves the way the writers gradually are incorporating her own quirks into Molly's character.

That "singing to her food" business? The writers didn't have to make that up.