"By day two I was like, ‘Oh my god.' It was so unbelievable. I said, 'This is it, this is it,'" the Big Bang Theory star said.
But Cuoco spent a much longer time -- two years -- secretly dating her co-star Johnny Galecki, telling CBS Watch! in 2010 that they wanted to protect themselves and the show by hiding their affair.
"It was a wonderful relationship but we never spoke a word about it and never went anywhere together," she said.
Asked to comment on their quiet relationship, Galecki said recently that he's still close with his ex.
"We're dear friends, still. Kaley's not just an ex, she's a part of my life," Galecki told CBS Watch magazine's December issue (via US Weekly).
Galecki said that he worried how fans of their Big Bang characters would respond to their real-life romance.
"I just don't like to speak about it. And not because I'm trying to be enigmatic; I just worry that it will conflict with people's acceptance of Leonard and Penny," he added. "I get the curiosity, but I don't want to distract from the story."
Galecki is now dating former Pan Am actress Kelli Garner.