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Kunal Nayyar, left, Jim Parsons, Simon Helberg and Johnny Galecki are the "geeks" in the hit CBS comedy "The Big Bang Theory."

BURBANK, Calif. — "The Big Bang Theory" bucks two current trends in television. First, it's a sitcom that's really, really funny. And, second, it's a sitcom that's really, really successful.

And there aren't many sitcoms that can make either of those claims anymore.

Co-creator/executive producer Chuck Lorre rejects the idea that he knows something very few others do these days, but the fact is that he's got two successful sitcoms on the air (including "Two and a Half Men") at a time when whole networks don't have even one.

"We just make shows that we love to laugh at ourselves and hope that somebody else out there agrees," Lorre said. "That's really about it. If it's not funny to us, we don't do it."

"I'll tell you this," interjects Bill Prady, the other "Big Bang" co-creator and executive producer. "The pleasure and pain of working with Chuck is that Chuck, every minute, on every script, on every page, in every moment, says two things: 'Do we believe the characters would do this?' and 'Why are we laughing?' "

The simple answer to why America is laughing at "The Big Bang Theory" (Mondays, 7 p.m., CBS/Ch. 2) is that it's funny. Like so many successful shows, the premise is actually quite simple — a variation on the classic fish-out-of-water story.

Leonard (Johnny Galecki) and Sheldon (Jim Parsons) are brilliant physicists/roommates who struggle with the real world. Sheldon, in particular, can't comprehend how "normal" people operate.

They hang out with their buddies/fellow geniuses, Howard Wolowitz (Simon Helberg) and Rajesh Koothrappali (Kunal Nayyar), playing and Klingon Boggle. But when a beautiful blonde, Penny (Kaley Cuoco), moves in across the hall, Leonard wants something more. Maybe even a relationship.

"Leonard is the only character that's in motion by his own choice," Prady said. "He is the only one who is reaching for something."

"That was apparent to me early on — that he just had this nagging notion that other people have fuller lives than him," Galecki said. "He just doesn't know how to attain that. There's no manual, and he has not been given an invitation outside of the world that he lives in, and it's frustrating. So there's this kind of fun dichotomy to play — that frustrated impotence, and yet he's a genius."

The show's breakout character, however, is Sheldon. He's often insufferable, but he's utterly without malice.

"There's an innate charm and sweetness to Jim that it allows us to make him as obnoxious as we want, and we can get away with it," Lorre said. "There's an innocence to it that comes through."

"The somewhat unspoken collaboration between us — me, Chuck, the rest of the writers — is simply that we have to walk up to the line. We cannot cross it," Parsons said. "And we've had to go through scenes where we've had to correct on the spot going, 'We've gone too far. We must bring this back. The audience is not pleased with the way you are talking to Penny. We can be biting, and he can observe something in a situation, maybe get snarky about it, but it can't be malicious.

"And it's a fine line. It is also the words — carefully chosen words of what he can say, especially to someone like Penny that you don't want to put it in a position where people are groaning. Well, not always, at least."

"Big Bang" has grown considerably in its relatively short run — 32 episodes and counting. A big part of that is because, as Lorre promised when the show began, the character of Penny is not just a dumb blonde.

"She's not just the cute girl next door," Lorre said. "She brings a little baggage, a couple of skycaps to get her to the curb."

"Yes, she's got a lot of issues," Cuoco said. "She's definitely not perfect. She has a lot of stuff going on, too, which is cool.

"It did start out as the girl next door, but they've been writing some really great, fun stuff for me. She's a real girl. She's smart in her own way, and I think I represent the audience. Like, I'm looking at them through your eyes because they are so different than what we are all used to."

The show does not, however, mock the smart guys. Even though Sheldon is both the smartest and most misanthropic of the bunch.

"I've always been against the whole idea of just calling them 'nerds' because it doesn't define who they are," Lorre said. "They are human beings. They have parents, and they have brothers and sisters and goals and fears and …"

"And they are geniuses," Parsons interjected.

"It doesn't really give you any insight into them by giving them a name of 'nerd' or 'geek.' It doesn't begin to describe what they are," Lorre said. "In fact, they are probably the characters who will change the world.

"They may blow it up. That will be the change."

Lorre and Prady make a decided effort to make their geniuses funny without ridiculing them.

"From the beginning, we've had great respect for our characters and a desire to have their world be as real as you can make it in a comedy," Prady said. "But the things they are doing in there are real, and they are not stereotypes."

They have a consultant "for the hard science," Prady said, but they don't really need a consultant to make the, well, geeks (for lack of a better term) seem real.

"I think if you spend some time in the writers' room, you'd understand," Prady said. "Our writing staff will go to 'Star Trek' references all day with any group you bring on."

And the real-life counterparts to Leonard, Sheldon, Howard and Raj are among the show's biggest fans.

"I've got to say that's been one of my favorite things about this is just the enthusiastic response that we've gotten," said Prady.

As a matter of fact, George F. Smoot, the 2006 Nobel Prize winner in physics — whose work has helped confirm the actual Big Bang theory — wrote the producers a fan letter and asking if he could make a cameo appearance on the show.

(He's in the episode scheduled to air on Monday, March 9.)

Of course, great writing alone is not enough to guarantee success. The lines have to be spoken by the right actors, and Lorre credits his "phenomenal cast" for making "Big Bang" a hit.

Certainly the undeniable chemistry between Leonard and Sheldon owes a lot to the rapport between the two actors.

"When Johnny and Jim read together, you knew it. You just knew it," Lorre said. "You'd sit there and say, 'Oh, OK. We've got a show.' "

Parsons said the chemistry between him and Galecki was immediate.

"We literally didn't know each other when we read together the first time, and it just felt different," he said. "We had read with several other people, and it was a marked difference."

Galecki recalled meeting Parsons for the first time outside the audition.

"And I said, 'Do you want to rehearse it? Do you want to go through it once?' " Galecki said. "And he said, 'No.'

"And I said, 'Perfect. I like you already.' "

The fact is that "Big Bang Theory" didn't work the first time around. Lorre and Prady produced a pilot that CBS rejected in 2006.

"We didn't know what we were doing," Lorre said. "It's a lot of trial and error. The first pilot we wrote for the show was wrong. I don't know any other word to use for it."

They reworked it from top to bottom, keeping only Galecki and Parsons.

"We created the characters for Simon and Kunal, and we remodeled the female character so that Kaley could come in and knock it out of the park," Lorre said.

Lorre credited CBS executives for giving the show a second chance and paying for the second pilot in 2007.

"I don't generally throw flowers at them, but when we struggled a bit to get this thing right, they gave us another chance because of Johnny and Jim. I mean, the chemistry between them was undeniable," he said, comparing it to the pairing of Thomas Gibson and Jenna Elfman on "Dharma & Greg" and Christine Baranski and Cybill Shepherd on "Cybill."

"And the same thing happened with every other character," Lorre said. "When Simon and Kunal and Kaley walked into the room, I went, 'Oh, OK. Thank you. This is a gift.' "

And there's an amazing humanity to the show. The series' Christmas episode was a highlight, when Sheldon was so stunned by Penny's thoughtful gift that he actually hugged her — awkwardly, of course.

"It was a beautiful story," Lorre said. "I love that one. It's one of my favorites."

Both Cuoco and Parsons quickly agreed.

"It was really so fun to get to do something where Sheldon was — I mean, not only was he rendered speechless, which is a rarity, he was pushed to physicality to express an emotion he was having," Parsons said. "I mean, my (gosh), that's as close he's had to a sex scene."

Just don't expect any of these characters to change overnight. Part of the formula of keeping a successful sitcom successful is to keep the characters the same while, at the same time, allowing them to grow "oh-so slowly," Lorre said.

"All baby steps for these guys," Galecki said.

"All baby steps," Lorre agreed. "Yeah, if there's any magic trick to sitcoms — stuff happens. Nothing changes."

Just the way nothing much has changed about the sitcom format.

"If you love these characters and you are rooting for them, then situations take care of themselves," Lorre said. "So, in essence, nothing has changed. It's the same storytelling. … We put on a little play. Look, it's a couch. And then there's a kitchen. There's no bells and whistles. There's nothing we can do in editing to save it. It's right here, and this is what the audience sees (at a taping) on Tuesday night.

"And if we tell a good story about these people, maybe it's worth watching."