Melissa Rauch as Bernadette Rostenkowski-Wolowitz (recurring season 3, starring seasons 4–12): A young woman who initially is a co-worker at The Cheesecake Factory with Penny to pay her way through graduate school, where she is studying microbiology. Bernadette is introduced to Howard by Penny; at first, they do not get along, apparently having nothing in common. They date and later get engaged and married. Although generally a sweet and good-natured person, Bernadette has a short fuse and can be vindictive and lash out when provoked.
Showrunner Steve Molaro, who took over from Bill Prady with the sixth season, credits some of the show's success to the sitcom's exposure in off-network syndication, particularly on TBS, while Michael Schneider of TV Guide attributes it to the timeslot move two seasons earlier. Chuck Lorre and CBS Entertainment president Nina Tassler also credit the success to the influence of Molaro, in particular the deepening exploration of the firmly established regular characters and their interpersonal relationships, such as the on-again, off-again relationship between Leonard and Penny. Throughout much of the 2012–13 season, The Big Bang Theory placed first in all of syndication ratings, receiving formidable competition from only Judge Judy and Wheel of Fortune (first-run syndication programs). By the end of the 2012–13 television season, The Big Bang Theory had dethroned Judge Judy as the ratings leader in all of syndicated programming with 7.1, Judy descending to second place for that season with a 7.0. The Big Bang Theory did not place first in syndication ratings for the 2013–14 television season, beaten out by Judge Judy.
Although the original pilot was not picked up, its creators were given an opportunity to retool it and produce a second pilot. They brought in the remaining cast and retooled the show to its final format. Katie was replaced by Penny (Kaley Cuoco). The original unaired pilot has never been officially released, but it has circulated on the Internet. On the evolution of the show, Chuck Lorre said, "We did the 'Big Bang Pilot' about two and a half years ago, and it sucked ... but there were two remarkable things that worked perfectly, and that was Johnny and Jim. We rewrote the thing entirely and then we were blessed with Kaley and Simon and Kunal." As to whether the world will ever see the original pilot on a future DVD release, Lorre said, "Wow, that would be something. We will see. Show your failures..."
Although earlier this season Penny (Kaley Cuocuo) told her husband Leonard (Johnny Galecki) that she never wanted kids, part two of the finale reveals that she is pregnant. But when Sheldon dismisses her announcement, Leonard angrily says “to hell with” his Nobel Prize. When his friends threaten to leave at his poor treatment of them, Sheldon takes time to self reflect.
Sheldon and Amy's wedding day arrives, with many friends and family attending, including Amy's domineering mother (Kathy Bates) and quiet father (Teller); Sheldon's mother Mary, brother George, and sister Missy, who is pregnant with her second child and separated from her husband; and Mark Hamill, who replaces Wil Wheaton as officiant as a favor to Howard who returned his lost dog. Sheldon cannot get his bow-tie to be symmetrical and conversations with Amy and his mother give him an epiphany about superasymmetry. Sheldon and Amy, along with Leonard, get caught up figuring out the math of the theory during the wedding ceremony, making Hamill take Star Wars questions to stall for time. Penny retrieves the couple, who then decide that they have the rest of their lives to make science together. Amy and Sheldon then express their vows. Amy recites a set of heartfelt vows, leaving Sheldon initially at a loss for words, before he reciprocates her loving words. They are pronounced as husband and wife.
The show originally centered on five characters living in Pasadena, California: Leonard Hofstadter and Sheldon Cooper, both physicists at Caltech, who share an apartment; Penny, a waitress and aspiring actress who lives across the hall; and Leonard and Sheldon's similarly geeky and socially awkward friends and co-workers, aerospace engineer Howard Wolowitz and astrophysicist Raj Koothrappali. Over time, supporting characters were promoted to starring roles, including neuroscientist Amy Farrah Fowler, microbiologist Bernadette Rostenkowski, physicist Leslie Winkle and comic book store owner Stuart Bloom.
However, no legal action was required to end production of the other show: as soon as it became known that the show was unlicensed, the actors quit and the producers canceled it. Dmitriy Tankovich (who plays Leonard's counterpart, "Seva") said in an interview, "I'm upset. At first, the actors were told all legal issues were resolved. We didn't know it wasn't the case, so when the creators of The Big Bang Theory started talking about the show, I was embarrassed. I can't understand why our people first do, and then think. I consider this to be the rock bottom of my career. And I don't want to take part in a stolen show".