When “The Big Bang Theory” debuted in 2007, it was easy to dismiss it as a one-note samba better titled “Two Geeks and a Gorgeous Chick.”

The sitcom may have started with a quartet of nerds who were more comfortable interacting with a Bunsen burner than a hot blonde, but it quickly began integrating a subset of characters that successfully pushed the show out of its comfort zone.

The result is a series that is one of TV’s highest-rated shows that can expect a lifespan to reach far beyond tonight’s 100th episode.

“It’s always a risk when you expand the ensemble that your main (characters) will be pushed to the sidelines,” says USA Today TV critic Robert Bianco. “But the bigger risk is what happens to characters like Sheldon (Jim Parsons) when left at the center too long. Broad characters are funny in small doses, but become tiresome when too much weight is put on them.”

Co-creators Chuck Lorre and Bill Prady tinkered with the series from the beginning, listening to the rhythms of the characters to see where the stories would go.

In addition to the central cast of characters — played by Parsons, Johnny Galecki (Leonard), Simon Helberg (Howard), Kunal Nayyar (Rajesh) and Kaley Cuoco (Penny) — the writers have brought in girlfriends for the boys who have certainly upped the laugh quotient.

Helberg’s on-screen fiance, Bernadette, played by Melissa Rauch, and Parson’s platonic girlfriend, Amy, played by Mayim Bialik, slowly took on larger roles in the series as the core ensemble blossomed from five to seven regular characters.

It was a seamless transition, says Parsons, because it came about so naturally.

“These changes actually happened ahead of a need,” Parsons says. “Our characters were not bone dry and played out. So instead of waiting until that happened and we had to adopt a child or something, they tried out characters for fun.”

Parsons says bringing in Bialik gave him the opportunity to play another side to Sheldon.

His character views Bialik as his intellectual peer — if not his equal — and respects her opinion.

“The relationship has allowed Sheldon to grow and he opens up, slight as it may be, to be more sensitive to others,” says Parsons, citing the bridesmaids episode when he comforts Bialik after she’s been shut out of a shopping trip with Rauch and Cuoco.

“I think about how grateful I am to have these scenes to play, and how refreshing for the actors and the series,” he adds.

Bialik says for many episodes during the series run, Cuoco’s character has been intellectually inferior to the boys and that scenario might have gotten stale. However, when she befriends Bialik and Rauch, “She gets to be the queen bee who can teach these nerdy academic girls things they don’t get.”

Despite adding characters and plotlines that might have been unheard of when the series launched in 2007, Bialik says Lorre and Prady never forget what makes “The Big Bang Theory” a winning formula.

“The show is really about two nerdy guys with a generic mainstream person, and the humor comes from the juxtaposition,” Bialik says. “That hasn’t changed. Amy and Bernadette just allow you to see them a little differently.”

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