Peter Cullen, the voice of Optimus Prime for years, remembers how he auditioned for director Michael Bay to play the character in Transformers. The film introduced the characters of the cartoon series to a completely new generation, revitalizing the '80s franchise for the 21st century. The action movie is celebrating the 15th anniversary of its 2007 release, which has since spawned many sequels as the characters and their world continue to grow.

Transformers follows the Autobots, led by the wise and noble Optimus Prime (Cullen), in their quest to find the AllSpark to rebuild their home planet of Cybertron. Fighting against them are the evil Decepticons, led by the villainous Megatron (voiced by Hugo Weaving), who want to use the AllSpark to build an army of machines on Earth. The alien robots fight and hide in plain sight by transforming into modern machines, mostly vehicles. Shia LaBeouf stars as Sam Witwicky, the teenager who gets caught up in the midst of the battle. Transformers also features Tyrese Gibson, Josh Duhamel, Anthony Anderson, and Megan Fox.

Related: Why Michael Bay's Transformers Movies Were So Popular (Despite Being Bad)

To celebrate the 15th anniversary of Transformers, Paramount Studios invited media members, including CBR, to tour the studio and see props from the film. Part of the event saw Cullen standing in front of Optimus Prime's big-rig form and sharing memories about his time working on the movie, including his experience auditioning for his famous role. Voicing Optimus Prime and Ironhide in the television series did not guarantee him either part in Bay's project. Read his comments below:

"When I first auditioned for Michael, a young gal was going to read the lines of the other character. We looked at the script, and it said Ironhide. I actually played Ironhide in the cartoon series as well. So I said, 'Well, I'd like to do a take of Ironhide, too.' Michael looked up and said, 'You can do that?' I said, 'Yeah, all the time. Frank Welker, one of the great voice actors of all time, he'd do Megatron and like five other voices. So we always ended up talking to ourselves.'... [Bay] always had a really big smile on his face. It was just interesting to see how [my voice] was going to fit how big Michael's concept of a 45-foot statue, how it was going to fit my voice. He was like, 'How are we going to make that adjustment?' So I said, 'We'll just go lower.'"

While Cullen did not voice Ironhide in Transformers—that role went to Jess Harnell—his versatility impressed Bay. After two more auditions, Cullen did get the part of Optimus Prime, and the task of making his voice fit the massive robot began. As he said, they adjusted the voice lower to fill the size of the character. It clearly worked very well, and Cullen has remained involved with the franchise ever since, reprising his role as Optimus Prime in each subsequent sequel.

Since Cullen is the originator of the Optimus Prime character, it is funny to hear that he had to audition at all since it is almost impossible to imagine anyone else in the role. However, the seasoned voice actor did audition, and he won the part. Truly, his performance is a standout in Bay's Transformers and its sequels, so it was certainly the right decision to let Cullen reprise his cartoon role.

Next: What Happened To Megan Fox's Mikaela After Transformers 2

Source: CBR