"The Big Bang Theory" may now be gearing for its ninth season and while viewers are tuning in to its premiere to find answers from the cliffhangers from last season. There are just moments from last season that we can't just leave behind yet and one of which was a surprising celebrity appearance by Billy Bob Thornton and know what's behind his short appearance for the sitcom.

The "Fargo" actor that happened to pass-by the nerd comedy sitcom wasn't accidentally casted for a role or first considered for a character. According to the article of TV Line, Thornton made the stint for his mother who is an avid fan of "The Big Bang Theory."

Thornton also said according to the article, "I said in an interview that my mom's favorite show was 'The Big Bang Theory'," and he also said, "So [exec producer] Chuck Lorre got a hold of my manager and said, 'Would you consider doing a guest spot for your mom?'"

Thornton starred in the eighth season of the sitcom for its seventh episode entitled "The Misinterpretation Agitation." His short stint role was portraying a Urologist Dr. Oliver Lorvis who was smitten by Penny and wants to start a romantic relationship with her.  

Aside from Thornton, some of the guest stars on the eighth season of "The Big Bang" theory were Stephen Root, Nathan Fillion,  Kara Luiz, LeVar Burton, Jim Meskimen, Stephen Hawking, Matt Bennett, and Sara Erikson. Who do you think will probably guest stars again on the ninth season of "The Big Bang Theory?" 

Some tidbits were given on the upcoming TBBT season 9 premiere of the sitcom and viewers may expect that there will be some confrontation that will happen between Sheldon and Amy. Meanwhile a hanging question is left for Penny, if she will continue to wed Leonard even if she knew of his indiscretion.

Watch what happens on the ninth season of the "The Big Bang Theory" premiere episode set on Sept. 21.