From BBC News' article:

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.