Here he is telling a powerful story about finding the voice by being inspired by his brother's calm authoritative voice, a Vietnam veteran. https://www.youtube.com/watch?v=zryfjSaxXLo

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horacemorace 2 days ago | parent | next [-]

“Strong enough to be gentle” is the epitome of manhood.

	▲	gchamonlive 2 days ago \| parent [-]
		Uncontrolled power is easy, just give a kid a gun, real power is serene. "All truly strong people are kind" - Vagabond

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proactivesvcs 2 days ago | parent | prev [-]

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.