His Optimus Prime voice was inspired by his older brother, a decorated US Marine and Vietnam War veteran, who suggested he should play the character as "a real hero, don't be a Hollywood hero".

"He said, 'Don't go yelling and screaming. He's strong enough to be gentle.'

"And that resonated with me, and I could hear his voice all the way to the audition. When he drops down, when he gets serious, is where I found Optimus Prime."

Cullen gave his brother a cut of the wages he received for playing Optimus Prime until his death.

The neat thing about characters is they can outlive the actors who play them. Retiring Optimus would be like retiring Bugs Bunny after the death of Mel Blanc.