WITH AN EYE ON . . . : An impish Charlie Schlatter provides this season’s Rx for “Diagnosis Murder

Diagnosis: Stuffiness. Rx: Levity in the “medicinal” form of impish charmer Charlie Schlatter.

Producers hope that Schlatter will make Dick Van Dyke’s Dr. Mark Sloan “less stuffy” as the CBS’ series “Diagnosis Murder” moves into its third season, which began last month.

Executive producer and director Tom Chehak says he wants to bring Dr. Sloan “back to what he was originally created as: a kind eccentric, bumbling character who’s distracted by his thoughts. Now, with Charlie as intern Dr. Jesse Travis, we’ve been able to capture a more carefree spirit with Dr. Sloan. Jesse feeds Dick’s character, because in actuality, he is Dr. Sloan at that age.”

“Charlie has a marvelous energy,” says Chehak. “And a wide-eyed innocence that helped us with the younger audience.”

Schlatter, 29, who last appeared in Fox’s pilot for the currently on-hiatus “Too Something,” replaces the now-departed Scott Baio. Of Jesse, Schlatter says with a laugh, “He’s basically me. He’s very goofy.”

Schlatter’s self-effacement is no act. Although extremely chatty over a lunch of grilled cheese and fries, Schlatter doesn’t find it easy to talk about himself. Luckily, joining in the lunch is his wife, Colleen, who happens to be a publicist, and as a matter of fact, is his publicist.

It’s Colleen who proudly points out that “Charlie can play just about any instrument and sings too,” reminding that New Jersey-native Schlatter has a musical theater background, which began in seventh grade. He received a bachelor of fine arts in musical theater from Ithaca College.

Schlatter’s career once seemed on a near-meteoric rise with his first role in 1988’s “Bright Lights, Big City,” followed by leads in 1988’s “18 Again!,” 1988’s “Heartbreak Hotel,” 1990’s Australian blockbuster “The Delinquents,” 1992’s “All-American Murder,” and 1992’s video release “Sunset Heat.”

“A lot of good projects got away,” Schlatter says matter-of-factly. He acknowledges he may not have made the best choices in film roles. His follow-up films to “18 Again!” were disappointments in the U.S.

Even Schlatter’s turn in NBC’s 1990 sitcom “Ferris Bueller,” based on the Matthew Broderick film, proved ill-fated. The series lasted only four months.

“When I had a chance to be the one to say, ‘Yes,’ ” Schlatter recalls, “I was waiting for that one project. The ‘one.’ ” When it became apparent that the role of a lifetime wasn’t coming his way, it was too late. “By that time, other people were being asked to say ‘yes.’ ”

But Schlatter has no regrets. “You get to know your neighbors real well when you’re not working and outside gardening a lot,” he jokes of their San Fernando Valley hillside neighborhood.

The actor’s clear about what’s important: “Doing what you love, which includes acting. I haven’t been a snob about TV or film or theater. I’ve kept a hand in all of them. Here’s what is most important,” he says pointing across the table: “Colleen and any children we might have.”

Lest the moment become too sentimental, Schlatter flashes his crinkly-eyed smile and says he hopes to eventually direct and see one of his scripts produced.

Schlatter hasn’t abandoned features, either. With a co-starring role in Universal’s upcoming “Ed” starring “Friends’ ” Matt LeBlanc, he continues to toil on the big screen.

Working with an icon and an idol like Dick Van Dyke has been a dream for Schlatter. “It really has been just a pleasure and complete fun,” he says of the seven-day shoots per episode.

Chehak agrees: “Charlie’s a wonderful actor. From the seed of the character I created, he’s nurtured it into a full-grown tree. He’s very inventive, and has great comedic timing. He knows how to play the subtleties of a line. Best of all, he really plays well opposite Dick. There’s a great chemistry between the two.”

Schlatter hopes that chemistry plays itself into a song-and-dance number with his castmate and favorite song-and-dance man.

“Oh yeah,” says Schlatter, who looks forward to more musical theater. “They’ve already done something where there’s been a murder at a talent show, but maybe another opportunity will come up where we’ll get to do a little soft-shoe together. Now that would really be great!”

“Diagnosis Murder” airs Fridays at 9 p.m. on CBS.