In a special featurette released by CBS, the actor said she had tried out for the sitcom's original unaired pilot, but failed to land the role of a character named Katie, who sounds worlds away from Penny.
Cuoco described Katie as "a dark, brooding unhappy woman".
The role eventually went to Amanda Walsh, who is deemed the "pre-Penny female lead" of the successful sitcom.
"Chuck [Lorre, producer] called a year later and, 'It's a new character, we want you to come in,'" Cuoco added, referencing the upbeat Penny. "I read for it and it just felt so much better than it did the year before. I guess it was just meant to be."
Lorre reworked the entire show after CBS turned down its original pilot, which he called "a mess" due to the fact the team "didn't really understand the characters yet".
He added: "We had to go through that failure to understand that if a woman was going to be in [Leonard and Sheldon’s] ecosystem, she had to be gentle with them because they were really vulnerable."
Cuoco played Penny in all 12 seasons of The Big Bang Theory, alongside Jim Parsons and Johnny Galecki. The entire series is available to stream on Netflix.