In Britain, where a “season” of a show might easily be six carefully crafted episodes, it’s commonplace to see a Ricky Gervais write every script and star in every episode.

American TV, though, with its 20-plus episode seasons, has long adopted a tidy assembly-line approach: Writers work in the writers’ room, actors on the set.

Yet writer-performers are very much in evidence Stateside, especially in comedy, where the factory approach seems to be running out of steam.

Tina Fey, doing double duty as showrunner and star, has helped shape “30 Rock” into a rare survivor among new network comedies, while “The Office,” also on NBC, has at least four people wearing both hats: star Steve Carell, Paul Lieberstein, Mindy Kaling and B.J. Novak.

On the hourlong side, Denis Leary is a full-fledged writer-producer as well as star of “Rescue Me” for FX.

After more than 30 years on “Saturday Night Live” and its numerous movie spinoffs, perhaps no producer alive has more experience with comedy writer-actors than Lorne Michaels, who is working with Fey on “30 Rock.”

“What Tina does brilliantly is she knows how to lead a writing staff,” Michaels says. “The most growth this year has really been her as a performer, but she’s completely comfortable in a room with a writing staff.

“I think that she can sit there and talk in a dispassionate way about her own character and Alec and the need for Jack McBrayer’s character or Tracy’s character to be brought into this other scene. She’s not sitting in that room going, ‘What about my part?’ “

“The Office,” for its part, is adapted from one of those Britcoms. When Greg Daniels began work adapting it, he deliberately went after writer-performers. Novak, a one-time standup comic, recalls going up for the show precisely so he could do both.

“He wanted that ‘SNL’ vibe, where we’re all one comedy gang that’s making the show ourselves,” says Novak.

Daniels, who argues that more comedies would benefit from having writer-performers, says, “If you were just picking funny people to be your actors, you would find that funny people often can write funny stuff. But the TV business often has other considerations when they’re casting their comedies and they wind up with former models that aren’t all that funny.”

Whether the show is a comedy or a drama, hyphenates bring one advantage to the set: If there’s a problem with the script, there’s someone on the set who can fix it.

On the other hand, the hyphenates have a difficult line to walk when they’re performing.

Novak recalls once making the mistake of stopping an improvisation with another actor to say, “My line should be this and your line should be that.”

“I could tell that was the wrong thing to do as I did it,” he says. “As two actors in the scene, that’s unfair. If I were just a writer on the set, I would have no problem trying to think of a better line. But as an actor, to really be part of the ensemble and work well as a team, you can’t have that imbalance on the set.”

Lieberstein, who was strictly a writer before coming to “The Office,” finds his actor mind and producer mind in conflict in another way. Carell, he explains, will pounce (in character) on any actor who fumbles an improv.

“He just devours you. If you take too long, he makes fun of you. If you say something that didn’t follow, he makes fun of you. It’s kind of amazing. While I’m watching it as a producer, when I’m not in the scene, it’s definitely something I’m hoping for.”

But how does Lieberstein feel when it’s he and Carell on camera?

“I’m praying he just sticks to the lines.”

Such moments of discovery are a staple of “Rescue Me.” Leary, Peter Tolan and Evan Reilly write the episodes together, with Leary by his own account writing up to 70% of any script. But on the set, Leary deliberately sets up improvisations.

“It’s really about making the actors so comfortable that they’re not married to the words, they’re married to the emotions and the motivations that the characters have in the scene,” Leary says.

He points to a scene where his character, fireman Tommy Gavin, has a blowup with wife Janet, played by Andrea Roth.

They agreed in advance that Roth should say what her character needed to say. They agreed the scene could get physical, so he told her if things got out of control, she should grab his hair, as a wife might do in an argument.

“She used some of the words we’d written for her, but some of the stuff that came out was so unbelievably raw,” says Leary. “When we got done, I’d only heard about half of what she said because she was scaring me so much. But when we looked at the playback, I said, ‘That’s way better than what we wrote.'”

One concern that all these hyphenates share is that their days can be extremely long; simple fatigue can become an issue. Of Fey, Michaels simply says, “You just hope that she remains young and sturdy.”

Another concern is that when the actors are working, the writers’ room is shorthanded.

But it’s worth it, says Novak, for the freshness that the “comedy gang” feel provides. “I think surprise is really the greatest weapon in comedy and people could see through a manufactured product, and it’s less manufactured when people are doing it themselves. It’s like food that’s organic. Our comedy is right off the tree.”