Voice actor Peter Cullen has provided the voice for the leader of the Autobots for more than 25 years. His booming, deep tones have become so engrained with Optimus Prime and the 'Transformers' franchise that it's hard to imagine it going on without him behind the mic.

So where exactly did that commanding voice come from?

Cullen revealed the secret source of Optimus Prime's sound to NBC News' "InGame' blog.

He told the blog that the voice of Optimus Prime is actually his older brother Larry. Cullen said Larry served in Vietnam as a Marine Corps officer where he earned several purple hearts and a bronze star. He also looked up to him and described him as "the one who inspired me to be a leader without shouting."

When Peter got his first audition for Optimus Prime for the 1980s version of the animated series, he told his brother he was "auditioning for a truck." His brother asked him what kind of voice a truck would make as a joke and Cullen told him that he actually speaks and is seen as a hero and a leader to his fellow robots.

That's when Larry told him how he should audition for the part.

He said, 'Well if you are going to be a hero, Peter, be a real hero. Don't do all the flashy Hollywood stuff, do it from your gut.'"

That made him realize he should just impersonate Larry as the voice of Optimus Prime and a week later, he won the role. He calls that interaction with Larry as one of the "most important moments of my career."