You've heard of reality TV. And of course, you've heard of sitcoms. But what about a cross between the two?

"Fat Actress" (search), starring and co-created by "Cheers" star Kirstie Alley (search), is the latest example of a fledgling television genre called reality-based comedy, in which stars play themselves in shows based on their actual lives.

"They occupy a niche right between sitcoms and the half-hour slice-of-life reality shows — like 'The Anna Nicole Show' on E! and 'The Osbournes' on MTV," said New York Post TV columnist Adam Buckman. "These shows are greatly fictionalized, but they have the look of reality."

Along with "Fat Actress," "Notorious" (search), a show based on the life of TV heiress and actress Tori Spelling, and an as-yet untitled sitcom about "Scooby Doo" star Freddie Prinze Jr. (search) are also in the works.

But the show in the new genre getting the most buzz right now is "Fat Actress," which premieres Monday night on Showtime. The comedy is a fictionalized version of the life of 54-year-old Alley — the former "Cheers" and "Veronica's Closet" TV star who also played in the "Look Who's Talking" films opposite John Travolta.

In "Actress," Alley is a Hollywood has-been who's gotten too fat to get roles and spends her time trying to make a comeback and use her weight to her advantage.

Art truly is imitating life here. Alley hasn't had a major show since "Veronica's Closet," which ended in 2000. And as her character is asked to do on "Fat Actress," the once svelte "Cheers" star now appears in Jenny Craig commercials as a spokeswoman for the diet company.

But there are some differences between the real Alley and her TV counterpart.

"In the show, I'm much more tortured and much more desperate and worried than in my real life," Alley recently told The Associated Press. "But the exaggeration of that stuff makes it funny."

The program starring Prinze, who has signed a deal with ABC and Warner Bros. for the pilot, has been billed as a half-hour sitcom based on the 28-year-old "Scooby Doo" actor's life. Fiction is no stranger than truth in this case, either: The program's main character is a young Puerto Rican man raised in a household of women; Prinze's mother raised him in New Mexico after his father, actor Freddie Prinze ("Chico and the Man"), committed suicide in 1977.

As for the Spelling project, it, too, is a half-hour reality-based comedy. In "Notorious," the pilot of which has been picked up by NBC, the 31-year-old "90210" actress and daughter of TV mega-producer Aaron Spelling plays herself, a celebrity who's dogged by nasty supermarket-tabloid gossip. Her pet, a pug named Mimi La Rue, also stars in the show as herself.

"We poke fun at a lot of the tabloid rumors," Spelling told the New York Daily News last month. "There's so much good material there ... We'll see how it works."

Meanwhile, there's already a well-known success story in the reality-based comedy category. "Curb Your Enthusiasm" (search), which is thriving on HBO, stars and is about 57-year-old "Seinfeld" producer Larry David (search) as he fumbles through a life of wealth and fame in Beverly Hills. Along the way, David makes all kinds of gaffes and voices outrageous, politically incorrect opinions that many people might share but don't have the guts to talk about.

While both "Enthusiasm" and "Actress" cast actors as the real people in the stars' lives, some supporting characters also play themselves, as in the case of comedian Richard Lewis in "Enthusiasm," and Travolta and NBC executive Jeff Zucker in "Actress."

Another hallmark of the reality-based genre is that the shows are often at least partially unscripted. Both "Enthusiasm" and "Actress" are ad-libbed — though scenes are well outlined — and both have the "verite" look of home movies shot with a hand-held camera. The combination makes the programs feel more real.

"It's an unscripted comedy," Alley told FOX News' Greta Van Susteren Friday. "We get to improv and it's about people's neuroses … [But] there is a tight outline. You know what is going to happen in this story. You know where we're leading."

But the fact that significant portions of the shows are fictionalized or exaggerated provides for more creative license potential than ordinary reality shows, when producers and directors have to be clever editors to make the most of what they've caught on tape.

"In fictionalized reality shows, you can do more — you're not just dependent on whether you pick up something good on camera," Buckman said. "You can control it, guide it, manipulate the action in the old-fashioned way of doing a television show."

Buckman said the current trend is an offshoot of reality shows about celebrities. In addition to "The Osbournes" and "The Anna Nicole Show," MTV's "Newlyweds," about newly married pop singers Jessica Simpson and Nick Lachey, is another example, as are the competition reality shows that pit washed-up celebrities against each other.

"They are definitely influenced by the recent trend of creating reality shows around celebrities — frankly mostly B-level ones at that," said Buckman.

But the success of reality-based comedies with stars playing themselves is dependent not only on whether they're well made, but also on whether viewers are interested in the celebrities they revolve around.

Virginia stay-at-home mom Liz Haas — a fan of both celebrity culture and reality TV — said she'd tune in to shows that are about stars who pique her curiosity.

"I would watch it if I liked the star," said Haas, 35, of Falls Church. "If there was some parallel to their real life, I would definitely be interested."