BURBANK, Calif. -- Back in 2003, when the lead-off sitcoms on CBS's Monday night lineup were the inexorably awful "Yes, Dear" and "Still Standing," if anyone dared predict that the sharpest traditional, multi-camera sitcom in prime-time in 2009 would be on CBS, that person would have been laughed at or ignored by serious TV observers.

Geniuses are often ridiculed, as Sheldon (Jim Parsons) on CBS's "The Big Bang Theory" (8 tonight, KDKA) knows all too well. Yet today's best laughtrack-infused sitcom is indeed part of CBS's Monday night lineup.

Although "The Big Bang Theory" was greeted with some skepticism upon its debut in 2007 because the concept -- two nerdy roommates and the hot girl next door -- was reminiscent of "Three's Company," the show has emerged as a critical and audience hit.

During a January visit to the show's home on Stage 25 at the Warner Bros. lot, Parsons, whose literal-minded Sheldon has become the show's breakout star, said viewers find the "Big Bang" characters relatable -- up to a point.

"No one ever says they are just like Sheldon," he said. "Everyone always knows someone just like Sheldon."

Sheldon and roommate Leonard (Johnny Galecki), Caltech post-docs studying particle physics, both have stereotypically geeky interests (video games and science fiction), but Leonard is more adjusted. Sheldon, however, lives in his own world, seemingly unaware of normal social interactions that in the hands of a less skilled actor could render the character unlikable.

"There's an innate charm and sweetness to Jim that allows us to make him as obnoxious as we want and we can get away with it," said Chuck Lorre ("Two and a Half Men"), who created the show with Bill Prady. "There's an innocence to it that comes through."

For his part, Parsons said he and the writers have learned what level of socially obtuse behavior viewers will tolerate from Sheldon.

"We have to walk up to the line. We cannot cross it," he said. "He can be biting and he can observe something in a situation, maybe get snarky about it, but it can't be malicious. It's a fine line."

As the series nears the end of its second season, "Big Bang" has evolved beyond its simple premise. Nerdling friends Howard (Simon Helberg) and Koothrappali (Kunal Nayyar) have shown growth and gotten their own moments in the spotlight. Blonde neighbor Penny (Kaley Cuoco) is more than just the attractive girl next door, becoming something closer to one of the gang.

"I think she represents the audience," Cuoco said, noting that she's a stand-in for the way viewers at home see the guys. "They are so different from what we are all used to."

Well, most people, maybe, but not everyone. Producers said their most prized reviews of the sitcom were positive notices in Science magazine and a particle physics journal.

"I've always been against the whole idea of just calling [these characters] nerds," Lorre said. "It doesn't define who they are. ... They are probably the characters who will change the world. They may blow it up. That will be the change."

"Big Bang" didn't have the easiest birth. The show's first pilot -- featuring just Parsons and Galecki from the current cast -- was scrapped.

"The fundamental difference was the character of the woman," Prady said. "She was very tough and very prickly, and people didn't like her around our guys. Penny is much sweeter."

The set has remained constant in both pilots, with the guys' apartment decorated with an assortment of gadgets, action figures ("The female figures are very different now than when I was a child," Galecki noted humorously) and a robot that sits atop a card catalog with drawers labeled "Luke," "Vader" and "Solo."

"Since Sheldon is the neatnik, I thought he was likely to store things categorically and organize things," said set decorator Ann Shea. "Who knows what's in there?"

When "Big Bang" began, Parsons was an unknown quantity, while Galecki was familiar to viewers from his role on "Roseanne." Galecki, 33, said his performance as Leonard is inspired by actor Judd Hirsch, who in "Taxi" was the most normal character compared to those around him.

"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. Sheldon represents an absolute stubborn happiness with where he is."

Prior to "Big Bang," Parsons, 36, had done stage work, appeared in the movie "Garden State" and had a recurring role on "Judging Amy." Playing Sheldon has been a learning experience for the Texas native.

"I was geeky in a theater-type way. I don't know about comic books, and video games have never been part of my day-to-day life," he said. "It's a testament to the [show's] writing that once I say it, it makes sense."

"Big Bang's" writers often give Sheldon monologues that require Parsons to memorize long passages filled with technical terms.

"I don't know if it's gotten easier, but I've tried desperately to ease up on myself," he said, "because it can make you a little bunched up."

He's heard and read comparisons of Sheldon's social awkwardness to symptoms of Asperger syndrome, but the show's writers said Sheldon is Asperger-free, which came as a relief to the actor.

"It would be a lot of responsibility and it would put up some barriers," he said. "He is Aspergian, but that allows more freedom."

For the "Big Bang" writers and producers, success also allows more freedom and less network interference, but Lorre, a veteran of "Roseanne," "Cybill," "Grace Under Fire" and "Dharma & Greg," doesn't intend to push the form beyond its breaking point, particularly with regard to growing the characters too quickly.

"All baby steps," he said. "If there's any magic trick to sitcoms: Stuff happens, nothing changes.

"Can Archie Bunker get better? I don't think so."