In a teaser for an all-new episode of The Simpsons airing on Sunday, viewers see Marge leading a group of Springfield’s misfits through an audition. As the camera pans the stage searching for the source of a pitch-perfect voice, it surprisingly lands on Professor Frink, the nerdy scientist famous for his disastrous inventions and social ineptitude. While Hank Azaria is responsible for his speaking voice -- nailing the quintessential nerdy nasal tone -- the unsuspecting crooner’s singing voice is provided by multi-platinum performer and Simpsons superfan Josh Groban.

Groban described the role as a career highlight. "My life's work is now complete. Thanks Al [Jean], and [The Simpsons] for letting me be part of the good professor's time to shine," he wrote on Twitter.

My life’s work is now complete. Thanks Al, and @TheSimpsons for letting me be part of the good professor’s time to shine. APRIL 7! https://t.co/atPXQWvP07 — josh groban (@joshgroban) April 3, 2019

Of course, when it comes to guest-starring on the hit series, Groban is in good company. Outside of Hollywood, few places have seen as many celebrity visitors as Springfield. As the Emmy Award-winning cartoon continues its reign as the longest-running animated series and longest-running sitcom, the number of famous guests continue to rise. Season 30 kicked off with Wonder Woman herself, Gal Gadot, which was followed by appearances from Tracy Morgan, Billy Eichner, RuPaul and George Segal, reprising his role from Who's Afraid of Virginia Woolf? from 52 years earlier.

Earlier this season, ET caught up with some of the folks behind The Simpsons, and they dished about their all-time favorite visitors -- and whose appearances they're still vying for.

"In the episode where Homer goes to rock ‘n’ roll fantasy camp, we had Mick Jagger, Keith Richards, Tom Petty, Elvis Costello, Lenny Kravitz, all in one episode," producer Mike Scully said, referring to the season 14 episode, "How I Spent My Strummer Vacation." "It doesn’t get better than that."

He admitted that the team often writes roles for celebrities they’re personally dying to meet, as was the case he said for the golden era episode, "Homer at the Bat," which featured enough major leaguers to field a team.

For Pamela Hayden, the voice of Milhouse, her most memorable guest star was also a musician. "Gaga was fantastic," she said of the star's season 23 cameo. "People fall through the cracks and are struggling and she has done so much work for those people. It's hard for me to just love someone for their performance, I know it sounds Pollyanna-ish but I like to feel like I have to be a fan of the person."

FOX

No one has shared more scenes with celebrity guest stars than Nancy Cartwright, who voices Bart. The mischievousness 10-year-old has had over a dozen love interests -- most of whom were voiced by very famous leading ladies such as Reese Witherspoon, Sarah Michelle Gellar, Sofia Vergara, Natalie Portman, Zooey Deschanel and Anne Hathaway.

Cartwright's favorite? Meryl Streep, who voiced Bart's first girlfriend. "Meryl is iconic," Cartwright said. "I knew I was going to go in and do a private session with her and I was thinking, How is she going to do this? Because she can change her voice and do all of these accents and everything. We normally do four takes per each scene, she did like 10. She was fantastic. Each one was different. It was amazing."

And it seems the feeling was mutual. "You know what Meryl said to me?" Cartwright recalled with a laugh. "She said, 'My kids don't watch my movies because they're too young but they are huge fans of The Simpsons and I just want to ask you something. Can I have your autograph?'"

Pamela Hayden, Nancy Cartwright, Stephanie Gillis and Mike Scully Anthony Behar/FOX/PictureGroup

"A lot of guest stars say they do the show because of their children," writer Stephanie Gillis added. "I remember Richard Gere said that specifically.” At the time of the actor’s 2001 cameo, he was a father to a nearly 2-year-old son (coincidentally named Homer).

While it seems like celebrities are eager to become immortalized in yellow, there are some elusive high-profile people the team is still hoping to land. Gillis would like a U.S. president (former or sitting) to voice the show. Despite many satirical impersonations, The Simpsons has yet to have a president voice their own character. “They keep turning us down,” Gillis revealed. Cartwright, meanwhile, is vying for a visit from Hugh Jackman, who has declined previous invitations to appear in the past.

Luckily there will be plenty of more opportunities to snag some additional A-listers to guest star on the show. The series was recently renewed for two more seasons and the team told ET that they’d love to do another 30 years of the show. "I think of ourselves like SNL -- there is always good, new material and we have a great writing staff and obviously incredibly talented actors. So, why not keep going?" Gillis said.

The Simpsons airs Sundays at 8 p.m. ET/PT on Fox.

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