The main and supporting cast were the major reason for the Big Bang's lengthy success. However, guest stars also played a major role on the series as well. Countless stars appeared on the show, including the likes of Charlie Sheen, Billy Bob Thornton, Elon Musk, and so many other big names.

However, as we're going to reveal in the following, it took some convincing to land certain big names. One star in particular had initially rejected the show's offer, while another one needed a major pep talk from himself.

In the following, we're going to reveal how Johnny Galecki was able to lock in a major name on

. Although the actor usually seems to be quite reserved, he went all out in an effort to get an experienced actor to play the role of his father on the show. Galecki himself admitted that it was an aggressive approach, but it all worked out in his favor as the actor agreed to the cameo.

Certain Guest Stars On The Big Bang Theory Needed Convincing Before Accepting An Invite On The Show

Given the sitcom's popularity, it would seem like acquiring guest stars was made a little easier, especially with Chuck Lorre attached to the series.

However, given the massive names the show went after, it was made a little more difficult at times.

The show had plenty of memorable guest stars, but some were harder to lock down. Among them included William Shatner, given that he wasn't in favor of the creative aspect that was brought forward for his appearance.

"I’ve had several conversations with the creators, and what they wanted me to do, didn’t seem to be exactly right, so I said ‘find something that’s better,'" Shatner said. "They may have taken umbrage at that, I’m not sure."

The actor continues, "They wanted me to play myself a little obliquely, and they said it would work out, but I needed more assurance on that.

"

According to showrunner Steve Holland, things would take a turn later on once Shatner became familiar with Kaley Cuoco's work. Shooting a commercial with Kaley built a relationship between the two and the actor ultimately agreed.

Johnny Galecki Revealed With Stephen Colbert That He Approached Judd Hirsch And Asked Him To Play His Dad Without Even Knowing If He Watched Big Bang Theory

During candid interviews with the cast, Johnny Galecki usually appears to be the most reserved of the group, particularly when it comes to sharing behind the scenes stories. However, it was a different story when Johnny detailed his experience alongside Judd Hirsch.

Speaking with Stephen Colbert on the , Galecki revealed he had a complete fan moment with the actor, and didn't hesitate to offer him a role on the series. Looking back, Johnny realized his bold approach could have fell flat, given that he didn't even know if the actor knew the show, or for that matter, knew who he was...

The role pitched by Galecki wasn't just any role, he had a vision for Hirsch to play his dad on the show. Galecki retold the story and how it all played out between the two.

To Johnny's credit, he was able to land the major name. Hirsch also spoke out about the ordeal, and admitted to being 'cornered', by the actor.

Judd Hirsch Admitted To Being 'Cornered' By Johnny Galecki During The Actor's Big Bang Theory Proposal

. The actor shared that it all took place at at a celebration of Jamie Burrows' 1,000th show.

Hirsch reveals, "He just sort of cornered me one day. We were at a celebration of Jamie Burrows' 1,000th show, and at that celebration - since he did the pilots of all the shows that we know, he did The Big Bang Theory, believe it or not - and Johnny Galecki just came over to me on the stage when it was all over.

. . . 'Would you like to be my father?"

Hirsch would go on to reveal that he felt very surprised that Galecki had a vision of himself as his dad on the show. Hirsch wasn't entirely sure how that association came about, but he would ultimately agree to the cameo.

"I go, 'Why not? Why not?' And Johnny said, 'I never had a father on the show and I always thought that I was like you.' And I'm thinking, 'Where?' [laughs] He actually thought that, in real life, that he could have come from someone like me."

It made for another memorable moment on the show, and Hirsch was added to the list of impressive guest stars during 12 season run.