According to an upcoming Big Bang Theory oral history, fan favorite actress Kaley Cuoco nearly didn't nab a leading role at all.

The book reveals that the pilot originally featured a female lead called "Katie" who was the antithesis of Cuoco's Penny.

Instead of being a Midwestern optimist, Katie was "cynical, down on her luck, and off-putting." 

While Cuoco auditioned for the role, it wasn't given to her because she was too bubbly.

Instead, Jim Parsons read with actress Marisa Tomei while she was auditioning for the part. 

American Pie's Tara Reid and Saved by the Bell's Elizabeth Berkley went up for the role as well.

But the part of Katie ultimately went to Amanda Walsh after another actress, Jodi Lyn O'Keefe, did an unsuccessful table read.

Was thought to have brought a "darkness" to the role that didn't work, according to Valko.

And this isn't the only Big Bang Theory on-set secret that's been revealed by the upcoming oral history,

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