The Big Bang Theory is huge. Now in its sixth season, it reigns as the hottest sitcom in the US, and now hotter than ever with an average 19 million viewers each week there.
As almost every viewer knows, Big Bang centres on a gaggle of geeky Cal Tech scientists, chief among them Sheldon (Jim Parsons) and Leonard (Johnny Galecki), apartment mates who, by great good sitcom fortune, live across the hall from Penny, played by Cuoco. Penny aspires to be an actress but pays the rent by waiting tables at a Cheesecake Factory.
Unlike the boys, Penny is no genius. But neither is she a dumb blonde, which is key to the genius of Big Bang. "She's obviously not book-smart," says Cuoco, "but she's street smart and good-hearted and can make fun of herself. Maybe she doesn't always get what the guys are talking about. But in that way she represents most of the world."
Reflecting on what her hit sitcom has meant to other parts of her life, she mentions her parents - her father, a real estate agent who has weathered tough times, and her stay-at-home mum in Southern California.
Cuoco, who turned 27 last November, grew up happy to be the centre of attention.
"I was always silly and never took myself seriously," she recalls. "When my father had the camera out, I'd be up close and annoying. My father would keep saying, 'Move back! Move back'!" Her parents were supportive when she wanted to try acting, "but I always had multiple activities, so I never had to count on any one of them to feel successful."
She landed a role in a 1992 TV film starring Donald Sutherland, played Maureen McCormick in the 2000 TV film Growing Up Brady and landed jobs in lots of episodic shows. But by 2002, she hadn't worked in a year and was feeling discouraged.
Then her agent phoned about a project called 8 Simple Rules for Dating My Teenage Daughter. She auditioned for the part of the older daughter in a household whose loving but bewildered dad would be played by John Ritter. Cuoco got the role.
8 Simple Rules is remembered mainly as the final project of the beloved Ritter, who died abruptly early in its second year. "I just adored him," says Cuoco, unleashing a stream of memories of how he used to cut up on the set and how much she learned from him. "He'd put a potato chip on his shoulder and go, 'Do I have a chip on my shoulder'?"
Despite the fun, acting pushes Cuoco to question everything. "I wonder, 'Why did I do that line that way?' And I also constantly think I'm fat and hate my teeth. But I've gotten better over the years. I've started to accept." She smiles, revealing nothing remotely wrong with those teeth. "It's going to be fine."
The Big Bang Theory screens on TV2, Wednesdays at 8.40pm.