Watch The Official Big Bang Theory Podcast

Season four of "Big Bang" ended with Penny and Raj waking up in bed together. Showrunner Steve Molaro reveals why he spent the hiatus trying to figure out a way to undo it.

The legendary Josh Malina joins Jessica Radloff to look back at his recurring role as Caltech President Siebert, which began in Season 4 and continued through the show's final season. Josh reveals that he originally auditioned to play Leonard — before quickly learning he was too old for the part — and shares how "Big Bang" co-creator Chuck Lorre promised to find a way to bring him back someday as the guys' boss. Years later, that promise paid off.

Executive producer and showrunner Steve Holland joins Jessica Radloff to look back at how "The Big Bang Theory" evolved into one of TV's most memorable ensembles. Steve shares why there was no grand plan heading into Season 4 and reveals the episode when he realized the new seven-person cast had truly clicked. He also recalls how audiences first responded to Bernadette and Amy and why their additions changed the dynamic of the show. Plus, Steve reveals his least favorite episode of the entire series!

Simon Helberg reflects on the magic of "The Big Bang Theory" and the incredible way the show connected people around the world. Seven years after the series finale, Simon shares how his relationship with the show — and his castmates — has evolved. He also looks back at some of Howard's funniest moments from Season 4, including his hilariously unfortunate encounter with a robotic arm. Plus, Simon reveals how he feels about the possibility of a "Big Bang" reunion.

Melissa Rauch looks back on the season that changed everything as Bernadette became a series regular on "The Big Bang Theory." She recalls what it felt like to see her name in the opening credits for the first time, reveals how the producers discovered she could do the unforgettable Mrs. Wolowitz voice, and shares what she really thought about filming all those bedroom scenes with Howard. Plus, Melissa explains why sharing the screen with Kaley Cuoco and Mayim Bialik became one of her favorite parts of the series.

Brian Thomas Smith joins Jessica Radloff to revisit his nine-season run as Zack Johnson, Penny's lovable but hilariously clueless boyfriend on "The Big Bang Theory." He shares behind-the-scenes stories from the fan-favorite Justice League Halloween episode, what it was like being the "new kid" alongside Mayim Bialik, and the kindness the cast showed him from day one. Plus, Brian reveals the storyline for an episode that was scrapped after Kaley's horseback riding accident—and what fans never got to see.

Mayim Bialik joins Jessica Radloff to look back on her first season as Amy Farrah Fowler on "The Big Bang Theory." She shares how Amy evolved over Season 4, the surprising ways she and her character are alike beyond their scientific backgrounds, and how she handled fans who initially opposed Amy and Sheldon's relationship. Plus, Mayim reflects on what it was like sharing the stage with Jim Parsons for so many years and watching one of television's most iconic partnerships come to life.

Kunal Nayyar breaks down Raj's Season 4 journey, from his bromance with Howard to Raj's unrequited feelings for Bernadette and waking up next to Penny in the finale.

Season 4 marked a turning point for "The Big Bang Theory" with the additions of Amy Farrah Fowler and Bernadette Rostenkowski, expanding the core cast to seven. Director Anthony Rich joins host Jessica Radloff to discuss the challenges — and unexpected rewards — of bringing the larger ensemble together. They also revisit fan-favorite episode "The Love Car Displacement," revealing how the memorable road trip came to life and sharing the hilarious scene that ultimately ended up on the cutting room floor.