"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.' "