Jim Parsons played everyone's favorite nerd: Sheldon Cooper, on the CBS sitcom The Big Bang Theory for 12 seasons from 2007 to 2019, with 279 episodes. The show we know and love, which follows the lives of a friend group navigating personal and professional relationships with a geeky twist, almost looked completely different.

When the show was first pitched and the pilot was written, it didn't star everyone's favorite girl-next-door Penny (played by Kaley Cuoco). Penny was called Katie, and many different actresses were considered for the role, one being Marisa Tomei.

Before The Character Penny Existed, There Was Katie

While fans have come to know and love Penny—the girl next door, who falls in love with Leonard Hofstadter (played by Johnny Galecki), the character was Katie, and she was much different than the bright, bubbly character.

Chuck Lorre, the show's creator and executive producer, wanted to name the character Penny. But, at the time, CBS had another pilot in the works with a character called Penny, so the writers had to change the name. Furthermore, the character Katie was nothing like Penny. Whereas Penny was outgoing, optimistic, and had that Midwestern innocence, Katie was dark, cynical, down-on-her-luck, and off-putting.

When talking about Kaley Cuoco's audition for Katie, the casting director—Ken Miller—said, "Kaley Cuoco went to the network and tested for the first pilot of Big Bang. She's so sunny and bubbly, and they just loved her, but she just didn't bring the dark qualities. She just wasn't the essence of Penny in that first incarnation."

Cuoco, who was only twenty years old then, looked too young for a character that was supposed to show more life experience. The nature of Katie was initially introduced in the pilot on a curb, crying over her breakup with her boyfriend while she talked on the phone.

But Cuoco did end up landing the role of Penny after the show's creator, writers, and producers realized the darkness of the Katie character wasn't working.

Marisa Tomei Auditioned For The Big Bang Theory With Jim Parsons

In the show's early stages, one thing was sure: Jim Parsons would be playing the character Sheldon Cooper. The book, The Big Bang Theory: The Definitive, Inside Story of the Epic Hit Series, was written by Jessica Radloff, who is a leading journalist for the sitcom and who interviewed the cast and crew numerous times throughout the show's run.

Radloff's coverage of The Big Bang Theory is believed to have helped the show garner popularity and further the connection between the show and the fans with her interviews of the cast and crew.

While speaking to Radloff for the book, Parsons' said: "I had the part of Sheldon for a long time before everything was settled." He also revealed that he had done an audition with Marisa Tomei. "I read with Marisa Tomei. Johnny was cast by then, but he was doing a play in New York."

Parson continues, "I had come back to read with her. I remember going to the snack room after we were done and telling Ken and Nikki [Valko, casting director], 'I've never been in someone else's audition before!' And they said, 'Well, she was kind of really auditioning you.' And I went, 'Oh, that makes sense.' But oh my gosh, what a different world that would have been."

When asked about her experience with the audition process and more context as to why it didn't work out, Tomei, while speaking to Insider, said, "I thought it was a great script, and that's why we went so far as to check it out. And it's such an iconic kind of setup, all those archetypes, the scientists, and then the kind of little hottie and the whole thing. I certainly didn't know it was going to go on that long. It wasn't really the right fit for me at the time, but I really did enjoy the script itself."

Tomei auditioned for the role of Katie—who would later turn into the character Penny.

Kaley Cuoco On What it Was Like Playing Penny On The Big Bang Theory

While speaking about auditioning for Katie, Kaley Cuoco said, "I loved the role, and I loved Chuck. I had worked with him when I was fourteen or fifteen on a pilot called Nathan's Choice. I know Chuck really wanted me for the role of Katie, and I was definitely bummed when I didn't get it, but I kind of have a way of moving on because you have to."

But the actress didn't have to move on from the entire project, just the character, because she was cast as the leading lady Penny. Cuoco played Penny for 12 years and said she would've continued for another 20.

Now that she's doing other projects since the sitcom ended, Cuoco said, "I get asked a lot, 'Are you trying to get away from Penny?' And the answer is not at all. I would've played Penny for 20 years."

The Big Bang Theory is currently streaming on HBO Max.