Ever since The Big Bang Theory, a creation by Bill Prady and Chuck Lorre, premiered on CBS in 2007, it's been a hit.

The sitcom spread over 12 seasons with a total of 279 episodes. The show had many guest stars over the years, including Billy Bob Thornton who played a urologist, Eliza Dushku of Buffy the Vampire Slayer and Angel fame, and the voice of Leonard Nimoy in Sheldon' dream. Some guest stars appeared as themselves too like James Earl Jones and Mark Hamill.

Somewhat ironically, one of the least popular guest stars of TBBT was cast in its spin-off Young Sheldon. Who was it?

With its talented actors, including Kaley Cuoco, Jim Parsons, and Johnny Galecki among others, The Big Bang Theory was an instant hit.

Though some argue it is overrated, there's no doubt that it has been immensely popular all over the globe.

The Big Bang Theory is a prime example of a series that isn't like any other out there, so it's no surprise that a spin-off was launched after the show ended.

Like many shows and sitcoms, TBBT had its fair share of guest stars – one of whom has been cast in Young Sheldon.

One popular figure that appeared twice on the show was Bill Nye, the Science Guy. He appeared in seasons 7 and 12. Other well-known figures include Buzz Aldrin, LeVar Burton, Bill Gates, and Carrie Fisher. But some fans didn't like one guest star – and this one now has a role in Young Sheldon as no other than Sheldon's father.

Jimmy Speckerman in TBBT

When Jimmy Speckerman appeared as a character in The Big Bang Theory, he was portrayed as one of the people that bullied Leonard at school (besides his sister, of course). In Season 5, the character, played by Lance Barber, appeared in one episode, and needless to say, that very episode is so not fans' favorite.

"The worst guest appearance which made the episode hard to watch = Jimmy Speckerman (now = Young Sheldon's Dad)," a Reddit user wrote.

Jimmy gets in touch with Leonard via Facebook and asks him to meet for a drink. Leonard, of course, is confused why his bully would suddenly be interested in meeting him and finds the decision difficult.

In the end, he decides to meet him so that he can tell him everything he couldn't say in school. As the episode unfolds, Jimmy is seen wanting help for something because Leonard is smart. He gets drunk and apologies but then has no recollection of doing so in the morning. Leonard stands up to Jimmy.

Though the actor Lance Barber is not playing Jimmy Speckerman in Young Sheldon, fans were still surprised that he was then given the role of Sheldon's dad. TBBT has cast actors in multiple roles before, so it isn't unheard of.

Barber's character was undoubtedly bad on TBBT, but people are generally enjoying his portrayal of George Cooper Sr. in Young Sheldon. It seems like the majority of fans can separate the actor from the character, thankfully!