When the third season of “House” wrapped, beleaguered medical fellows Foreman (Omar Epps), Cameron (Jennifer Morrison) and Chase (Jesse Spencer) were out. A more conventional series would have found ways to bring those lame ducks back to House’s fold come season four, but producers chose a bolder approach: Find new fellows.

“It was a natural progression,” says executive producer Katie Jacobs. “Foreman quits. Chase gets fired. Cameron resigns. I mean, how long can you stand working for House and subjecting yourself to that abuse?”

Still, the actors’ colleagues were taken aback.

“It was certainly surprising for all of us,” says Lisa Edelstein, who plays the doctors’ onscreen boss, Dr. Cuddy. “I was worried for Jennifer, Jesse and Omar — I didn’t know what was going to happen to those guys. But little by little, we’re all finding our footing.”

“I was shocked,” adds Robert Sean Leonard (Wilson). “When Katie said Jennifer, Jess and Omar were going to shift around, and they were bringing in new doctors, I thought, ‘Really? Well, OK.'”

For a moment, Spencer worried he might be fired. “It did cross my mind. I wondered if they were unhappy with us, but they reassured us that we weren’t going to get fired; it was for the show, and they wanted to change things up a bit.”

However, even before knowing what changes would be made, Epps was psyched.

“We’d been doing the same formula for three years straight, so I was excited to see how it was going to play out and where they were going to take it,” Epps says. “Being on the show is one thing, but I’m also a fan of the show. Audiences are pretty smart nowadays, and fickle — there are so many other ways to get entertainment that it’s hard to get their attention — so from that perspective, it was a great curveball for the audience.”

Edelstein hoped a larger cast might help reduce Hugh Laurie’s grueling schedule, “but ultimately, everybody has to ricochet off his character, so now he just ricochets off more people.”

Outsiders thought the idea was risky for a successful series, but Jacobs didn’t.

“It didn’t feel like we were changing things that much because the spine of the show is medical mysteries,” she says. “Then you have this fabulous character and how he interacts with other doctors is like no doctor you’ve ever seen. That’s the essence of the show.”

How they expanded the cast — House’s version of an elimination reality show — drew attention. The 40 or so actors became part of a real-life contest. Producers intended to hire two, but chose three new cast members — Kal Penn (Kutner), Olivia Wilde (Hadley, aka Thirteen) and Peter Jacobson (Taub). “The interview process and how he played that game seemed so ‘Houseian’ to us,” Jacobs says.

“It was certainly a bizarre experience,” Jacobson recalls. “Anything could happen. We knew it was going to be down to about five of us, so trying to figure out the math … was crazy.”

There was an upside, too. “It definitely helped (forge) a sense of camaraderie,” Penn says. It took half the strike-shortened season’s 16 episodes to narrow the field, temporarily relegating Morrison and Spencer to the periphery.

“The new team, obviously, is going to have more screen time than characters in other parts of the hospital,” Morrison says. “But they’ve done a good job of making us relevant when they do use us.”

Spencer appreciates that fans missed their characters. “It’s not like, ‘We’ve had too much of (Chase) chasing Cameron — can we have less of them, please?’ … When I do a scene now, I feel it has more purpose within the storyline.”

The changes brought Wilson love and a strong storyline. “The last two episodes of that season (combining a bus crash and a sudden fatal illness for Wilson’s doctor girlfriend, Amber) were very unlike our show,” Leonard says. “I was really lucky to get Anne Dudek playing Amber. I liked her from the moment I saw her work.”

Overall, the original cast feels the new co-workers fit in well.

“Just look at the scope of it,” Epps says. “Having more characters in the hospital makes that world bigger.”

Morrison adds that long-running shows should make for a more potent mix: “You’ve got to keep bringing more people into the world to spice things up and create more opportunities for conflict.”

In the end, Wilde hopes that “when people eventually look back on this transition — like when they’re watching ‘House’ DVDs in 10 years — they say: ‘That was cool. It was an unusual time to make a transition that extreme, but hey, it worked out and made for interesting television.’ “