Every Big Bang Theory Actor Who Appears In Stuart Fails To Save The Universe — Episode 3 Spoilers Ahead!

"The Big Bang Theory" may have ended in 2019, but its universe continues to expand.

HBO Max's "Stuart Fails to Save the Universe" reunites viewers with comic book store owner Stuart Bloom (Kevin Sussman), his girlfriend Denise (Lauren Lapkus), geologist Bert Kibbler (Brian Posehn), and physicist Barry Kripke (John Ross Bowie) — and their quest to avert a multiverse Armageddon brings them face-to-face with alternate-universe versions of characters viewers have come to know and love from "The Big Bang Theory." (Read TVLine's Season 1 review here.)

Below, we've rounded up every actor from the "Big Bang" franchise who appears on "Stuart Fails to Save the Universe." We'll continue updating this list as additional guest stars are announced or show up on screen (and, should any alumni from "Young Sheldon" or "Georgie & Mandy's First Marriage" eventually make the jump, we'll include them, too).

Kevin Sussman was among those who auditioned for the role of Leonard Hofstadter in the original, unaired "Big Bang Theory" pilot (as was future co-star John Ross Bowie, who read for the part twice).

When CBS ordered a retooled pilot the following year, creators Chuck Lorre and Bill Prady instead developed the role of Howard Wolowitz with Sussman in mind. But ABC declined to release him from his contract on "Ugly Betty," where he recurred during the comedy's first season, and the part ultimately went to Simon Helberg.

Sussman finally joined the sitcom in Season 2 as Stuart Bloom, the struggling comic book store owner who started out as a confident romantic prospect for Penny before spiraling into perhaps the series' unluckiest character. He went on to recur in 11 of the sitcom's 12 seasons, appearing in 84 of its 279 episodes, before finally getting a second chance as the star of "Stuart Fails to Save the Universe."

Lauren Lapkus made her "Big Bang Theory" debut as Denise, the Comic Center of Pasadena's new assistant manager, in Season 11's "The Comet Polarization." 

After accompanying Stuart to Sheldon and Amy's wedding, Denise returned the following season as Stuart's girlfriend. In her final appearance, Season 12's "The Maternal Conclusion," she invited Stuart to move in with her. 

All told, she appeared in just eight of the sitcom's 279 episodes.

Lapkus reprises her role — now as a series regular — in "Stuart Fails to Save the Universe."

Brian Posehn made his "Big Bang Theory" debut as geology professor Bert Kibbler in Season 6's "The Contractual Obligation Implementation," though the character initially went unnamed. Bert was formally introduced in Season 7's "The Occupation Recalibration" and went on to recur throughout the remainder of the sitcom's run, appearing in 15 of its 279 episodes.

Posehn reprises her role — now as a series regular — in "Stuart Fails to Save the Universe."

John Ross Bowie made his "Big Bang Theory" debut as fan-favorite physicist Barry Kripke in Season 2's "The Killer Robot Instability," immediately establishing the speech-impaired scientist as a thorn in Sheldon Cooper's side. He went on to appear in 25 episodes across 11 seasons before making his final appearance in Part 1 of the sitcom's two-part series finale in 2019.

Among the sitcom's recurring players, only Kevin Sussman appeared in more episodes.

Bowie reprises his role — now as a series regular — in "Stuart Fails to Save the Universe."

Josh Brener first appeared on "The Big Bang Theory" in Season 5's "The Flaming Spittoon Acquisition" as Comic Center employee Dale. He reprised the role in Season 6's "The Tangible Affection Proof."

He returns to the franchise in "Stuart Fails to Save the Universe," this time as an entirely new character: Trevor, one of Stuart's most frequent customers. He first appears in Episode 1.

Ryan Cartwright made his first (and only) appearance on "The Big Bang Theory" in Season 6's "The 43 Peculiarity," playing Penny's community college classmate Cole.

He returns to the franchise in "Stuart Fails to Save the Universe," this time as an entirely new character: Kyle, a daily patron of Stuart's comic book store. He first appears in Episode 1.

Penn Jillette made his "Big Bang Theory" franchise debut in prequel series "Young Sheldon," appearing opposite his longtime stage partner, Teller.

In the Season 5 finale, "A Clogged Pore, a Little Spanish and the Future," 12-year-old Sheldon (played by Iain Armitage) struggled to cope with the first signs of puberty. After getting his first pimple, Penn and Teller appeared as Acne Vulgaris and his silent sidekick, Pus.

As first revealed in the trailer for "Stuart Fails to Save the Universe," Jillette returns to the franchise, this time as himself in Episode 1's post-apocalyptic wasteland.

Magician and illusionist Teller made his "Big Bang Theory" debut in Season 11's finale, "The Bow Tie Asymmetry," as Larry Fowler, Amy's taciturn father. He reprised the role twice during the sitcom's 12th and final season.

He later reunited with longtime stage partner Penn Jillette in the Season 5 finale of prequel series "Young Sheldon," playing Pus, the silent half of the dermatological duo Acne and Pus, as 12-year-old Sheldon (Iain Armitage) grappled with his first pimple.

As first revealed in the trailer for "Stuart Fails to Save the Universe," Teller returns to the franchise, this time as himself in Episode 1's post-apocalyptic wasteland.

Kunal Nayyar becomes the first member of the original "Big Bang Theory" ensemble to appear in "Stuart Fails to Save the Universe," though the Raj audiences meet is far removed from the astrophysicist we last saw in the CBS finale. Held prisoner for years in Grand Caliph Barry Kripke's post-apocalyptic Pasadena, this alternate-universe Raj is the only one capable of repairing the quantum interference device that can restore the original timeline.

In a particularly cruel twist, this Raj has finally achieved the happy ending that eluded him in the original series — he even rode an elephant to his wedding — only for his wife to be devoured by a giant moth shortly afterward. Before he can put his scientific expertise to use, Raj is fatally shot by Kripke, becoming the spin-off's first major casualty in Episode 1.

Riki Lindhome made her "Big Bang Theory" debut in Season 2's "The Cooper-Nowitzki Theorem" as Ramona Nowitzki, a graduate student (and later doctor) with an unrequited crush on Dr. Sheldon Cooper (played by Jim Parsons).

Ramona resurfaced in the Season 10 finale, "The Long Distance Dissonance," where an unwelcome advance on Sheldon prompted him to fly across the country and propose to Amy (Mayim Bialik). She made her final franchise appearance in the Season 11 premiere, "The Proposal Proposal."

When Lindhome returns to the franchise in Episode 2, Ramona is a receptionist at Caltech in an idyllic alternate reality where a repressive AI regime keeps everyone in line via microchips implanted in their brainstems.

Christine Baranski earned four Primetime Emmy nominations for her recurring role as Leonard's acerbic mother, Dr. Beverly Hofstadter, who appeared in 16 episodes of "The Big Bang Theory" across nine of the sitcom's 12 seasons.

The moment Baranski appears onscreen in "Stuart Fails to Save the Universe" Episode 2, longtime fans will likely assume she's reprising Leonard's mom. Instead, the Emmy winner plays herself, starring in propaganda films used to indoctrinate prisoners at an AI regime's reeducation camp.

Kaley Cuoco becomes the second member of the original "Big Bang Theory" ensemble to appear in "Stuart Fails to Save the Universe" — though the Penny audiences meet is far removed from the aspiring actress-turned-pharmaceutical sales rep we last saw in the CBS finale. In an alternate reality ruled by a repressive AI regime, this Penny leads the human resistance, convinced that Stuart, Bert, and Kripke hold the key to defeating the AI and restoring the original timeline.

Lorre and Prady say Penny was a natural choice to lead the resistance, citing her "get-the-job-done" mentality and long-established ability to take charge when the situation demands it.

Brian Thomas Smith made his "Big Bang Theory" debut in Season 3's "The Lunar Excitation" as Zack Johnson, the first man Penny dated following her breakup with Leonard. The lovable himbo then recurred for the remainder of the sitcom's 12-season run, with one memorable revelation in Season 7's "The Thanksgiving Decoupling" revealing that he and Penny had drunkenly married in Las Vegas years earlier before eventually having the union annulled.

When Smith returns in Episode 2 of "Stuart Fails to Save the Universe," Zack serves as Penny's loyal lieutenant in the human resistance, helping abduct Stuart, Bert, and Kripke after their release from an AI regime's reeducation camp.

Melissa Rauch made her "Big Bang Theory" debut in Season 3's "The Creepy Candy Coating Corollary," when Penny set Howard up on a date with her Cheesecake Factory colleague Bernadette. Rauch was elevated to series-regular status in Season 4, and went on to appear in 209 (out of 279) episodes.

Seven years after the 2019 series finale, she reprised her role as the microbiologist in "Stuart Fails to Save the Universe" Episode 3, this time as a literal demon! "When they told me it was going to be an 80-foot-tall Bernadette... my goal height has always been five feet," Rauch told TVLine with a laugh. "So to get 75 extra feet to play with was thrilling."

Wil Wheaton made his "Big Bang Theory" debut in Season 3's "The Creepy Candy Coating Corollary," playing a heightened version of himself. Initially a frenemy of Sheldon, Wheaton eventually became a friend of Dr. Cooper and a frequent presence in the group's lives, appearing in 17 episodes across nine seasons.

It was first revealed in an HBO Max sizzle reel that Wheaton would resurface in "Stuart Fails to Save the Universe." He appeared in Episode 3's magic realm as a messenger for the Wizards Guild of America, where he engaged in an epic battle with Bert... and ultimately died.

Joshua Malina first appeared on "The Big Bang Theory" as the oft-mentioned President Siebert (first name unknown) in Season 4's "The Benefactor Factor." He returned for two additional appearances in Season 5 before recurring throughout Seasons 11 and 12, appearing in 13 episodes overall.

As revealed in a teaser trailer for "Stuart Fails to Save the Universe," Malina returns to the franchise, seemingly as a member of the United States Army. The full extent of his role remains under wraps.

New episodes of "Stuart Fails to Save the Universe" release Thursdays at 9 p.m. ET on HBO Max.