As Optimus Prime
Cullen recalled auditioning for the role of the robot character Optimus Prime at a casting house in Burbank, California, explaining that as he read Prime's character breakdown, he saw that it was "the opportunity of the year", and heeded his brother Larry's advice: "Peter, don't be a Hollywood hero, be a real hero. Real heroes don't yell and act tough; they're tough enough to be gentle, so control yourself." Cullen later learned from his agent, Steve Tisherman, that he not only won the part of Prime, but also, to his surprise, the role of Ironhide as well, which he saw as a "home run".
He stated that Optimus was his favorite voice role, and that he based the voice of the Autobot leader on his older brother Larry, who served in Vietnam. "When he came home, I could see a change. He was quieter and he was a man and a superhero to me," says the actor. "I watched him and listened to him. I'd never had an opportunity to do a superhero, and when that came, [that voice] just came right out of me and I sounded like Optimus."
Cullen also stated that he had no idea of Prime's popularity until the character's controversial death in the 1986 animated film, because the studio had never given him fan letters from children addressed to Optimus. The public backlash over Optimus's death surprised producers greatly. Children were leaving the theaters distraught because of the character's death. The writers temporarily revived the character for a single episode in Season 3 called "Dark Awakening". Initially, this was intended to be his final appearance, but after fan requests continued, "The Return of Optimus Prime", a two-part episode was produced. The original ending of "Dark Awakening" was altered in reruns to include a teaser about the return of the character, and indeed, Optimus returned once and for all to lead the Autobots for the final five episodes of the original American cartoon series.
He reprised the role of Optimus Prime in the 2007 Transformers live-action film, the sequels Revenge of the Fallen, Dark of the Moon, Age of Extinction and The Last Knight, Bumblebee and Rise of the Beasts and the video games based on the film series. Though he was only contractually obligated to voice Optimus up to The Last Knight, producer Lorenzo di Bonaventura stated in 2021 that Cullen was welcome to voice the character for as long as he wanted. His final performance as the character would be Optimus Prime: Awakening in 2026.
Cullen again reprised his role as Optimus Prime in the video games Transformers: War for Cybertron, Transformers: Fall of Cybertron and Transformers: Devastation, and in the television series Transformers: Rescue Bots, Transformers: Prime and Transformers: Robots in Disguise. His performance in the premiere season of Transformers: Prime earned him a nomination for a 2011 Daytime Emmy Award in the Outstanding Performer in an Animated Program category.
