In the third season of “The Big Bang Theory,” Penny — the show’s sole female lead up until that point played by Kaley Cuoco — finally got some girlfriends in the form of Bernadette Rostenkowski (Melissa Rauch) and Amy Farrah Fowler (Mayim Bialik). The two were a great addition to the series, but when it comes to great character dynamics, there’s no question that Amy’s initial girl crush — and borderline obsession — with Penny is particularly funny, and according to the show’s writers, it ended up elevating Amy’s character to new heights.

As executive producer and eventual showrunner Steve Molaro told Jessica Radloff in her book “The Big Bang Theory: The Definitive, Inside Story of the Epic Hit Series,” he was inspired to give Amy a character trait he understood on an innate level, and it led to a phenomenal on-screen bond with Penny. “Amy had a desperate craving for a social life that she never had, and I understand that,” Molaro said. “It was nice to be able to take some of those feelings and experiences from high school and have a place to put them with Amy. The desperation of wanting someone to think you were their best friend. And not only someone, but someone as cool and popular as Penny. I know that feeling. I’ve said those words that Amy said. So to have that outlet was extremely cathartic.”

“Initially, Amy was just an offshoot of Sheldon. But that didn’t work at all,” creator and showrunner Chuck Lorre said in the book, referring to the fact that Amy is introduced initially as a love interest for Jim Parsons’ lead character Sheldon Cooper. “So all right, it was a baby step. And then when she became enamored with Penny — to have a friendship with a woman like that — meant so much to her.” Lorre said that the show evolved because it was “seen through different eyes,” and it was due to Molaro’s decision to take the show to a personal level. “It opened the door to stories that wouldn’t have been told otherwise,” Lorre said.

So what does Mayim Bialik think of Amy’s girl crush on Penny, which defines her character for at least a little while at the beginning of Amy’s tenure on “The Big Bang Theory?” She loved it! Bialik — who once confessed that she has trouble watching “The Big Bang Theory” — said that she loved the narrative throughlines for a ton of reasons.

“Amy kind of falling for Penny was such a sweet sister romance,” Bialik said. “And also, really ahead of its time in terms of gender fluidity and sexuality. It was Amy saying, ‘I appreciate that this woman is beautiful!’ It’s so sweet! The girl crush was some of my favorite stuff that I got to do. And because I knew Amy had to often be all over Penny, I would often hold back in rehearsal just so Kaley didn’t have to rehearse the intensity with me, like me being up in her business all week.”

Bialik isn’t kidding about the intensity, considering that there’s an entire plotline where Amy has a portrait painted of her and Penny which she then insists hangs in Penny’s home (which is definitely the apex of her infatuation), but it’s all still pretty funny. Plus, as the show went on, Amy and Bernadette became essential parts of the story, thanks in large part to their bonds with Penny.

Amy is definitely a little intimidated by Penny and Bernadette when her character first starts appearing on “The Big Bang Theory,” but once she lets them in, the three form a real and solid friendship. According to Chuck Lorre, the girl crush on Penny is what lets all of that happen, and it also gave Amy’s character to focus on something other than her boyfriend all of the time. “It was a very one-sided infatuation,” Lorre said of Amy’s girl crush. He continued:

“Amy was desperate for a friend. Forget about a lover! She wanted a friend. And the fact that she focused on becoming friends with Penny and Bernadette enlarged the character in a wonderful, wonderful way. It also allowed us to go really slowly with the relationship with Sheldon. There’s a magic trick in getting a show to continue and to stay relevant; it has to evolve, but it can’t evolve. It’s sort of a paradox.”

As far as Kaley Cuoco is concerned, she loved having two main female counterparts on such a male-dominated series, and she felt like it was all incredibly organic to boot. “The beauty of having more seasons is bringing new people in, so bringing the girls on was a total natural progression,” the actress said in Jessica Radloff’s book. “I loved how it started with them dating the guys, and you just saw this whole show turn into something else, but very organically.”

“The Big Bang Theory” is streaming on Max now.