All in all, there’s really nothing old hat about Newhart

Audiences would be happy if Bob Newhart performed nothing but his classic comedy routines.

But Newhart would choke on a steady diet of nostalgia. At 75, he still sees fresh material lurking around every corner.

“It isn’t like you devote five hours a day to writing new material,” Newhart says. “I wish I was that disciplined. It’s just that you’re open to everything. Your mind is never off, even when you’re on vacation.”

He’s open to new challenges as well. Last month, Newhart made his first appearance on the most successful new show of the season, ABC’s “Desperate Housewives.” He returns tonight.

Executive producer Marc Cherry asked him to play Morty, the estranged boyfriend of the mother of Teri Hatcher‘s character (played by Lesley Ann Warren). After reading the script, Newhart eagerly hauled his deadpan delivery and trademark stammer to the Top 10 series.

“Well, I was obviously flattered,” he says. “This is the hottest show on television right now, so they could have called up just about anyone in the world and asked him to play Morty. They could have called Bruce Willis, and he probably would have accepted.

“What attracted me was that it’s just plain, flat-out funny. The show is written on different levels, but, to me, it’s satire on a genre. At this point, I can’t see how anyone could suspect it isn’t satire.”

Newhart was working as an accountant in Chicago when his first album, 1960’s “The Button-Down Mind of Bob Newhart,” won a Grammy and made him an overnight comedy sensation.

That led to “The Bob Newhart Show,” an NBC variety series that earned an Emmy but was canceled after one season.

He hit TV pay dirt with the second incarnation of “The Bob Newhart Show” (1972-78), the situation comedy about Chicago psychologist Bob Hartley.

And he triumphantly returned with “Newhart,” the 1982-90 series about New England innkeeper Dick Loudon.

These days, appearing six to eight times a year on “Desperate Housewives” seems ideal to him.

Still, it’s a busy time for the comedian with the laid-back style and button-down mind.

The first season of “The Bob Newhart Show” was recently released as a DVD box set. He’s the subject of an “American Masters” profile PBS plans to broadcast in July, and he’s working on a memoir that Hyperion Books will publish in fall 2006.

“It’s not a kiss-and-tell book, because there’s no kissing to tell about, frankly,” Newhart says. “And memoir seems like a stuffy word, but it will be fun.”

He can’t help making the book funny. It’s what he does.

“I have a theory about not trying too deeply to figure out why you’re funny,” Newhart says.

“Maybe there’s a funny gene. I don’t really know. I just thank God it’s there, because otherwise I’d still be an accountant in Chicago.”

The birthday bunch

Comedian Don Rickles is 79. Singer Toni Tennille is 62. Actor Stephen Furst (“St. Elsewhere”) is 51. Actor David Keith is 51. Actress Melissa Gilbert is 41. Singer Enrique Iglesias is 30. Actress Julia Whelan (“Once and Again”) is 20.