For more than four decades, millions of people knew exactly what Optimus Prime sounded like.

Calm. Deep. Commanding.

He could lead an army without screaming. He could deliver a warning without sounding cruel. And even when the fate of an entire world seemed to be at stake, there was something unusually reassuring beneath the power in his voice.

That sound belonged to Peter Cullen, the actor who first voiced the heroic Autobot leader in the original Transformers animated series in 1984 and returned to the role again and again for decades.

But Cullen never considered Optimus entirely his creation.

The character’s voice — and even more importantly, his personality — came from someone Cullen had admired long before anyone had heard the name Optimus Prime.

His older brother, Larry.

Following Cullen’s death at age 85, the story behind that famous voice has taken on an even deeper meaning. Cullen died at his Los Angeles home on Wednesday, Aug. 26, according to his agent.

And remarkably, the words Cullen’s family chose to remember him echoed the same lesson his brother had given him before one seemingly ordinary audition more than 40 years earlier.

It Started With an Audition for a Truck

In 1984, Transformers was not yet the global entertainment empire it would become.

There were no billion-dollar movies, generations of devoted fans or decades of nostalgia attached to Optimus Prime.

For Peter Cullen, it was simply another voice audition.

He knew he was going in to read for a character that transformed into a truck.

Cullen later recalled telling his older brother Larry about the unusual assignment. According to Cullen’s retellings, the conversation initially seemed almost comical: he was essentially auditioning to play a truck.

Then Peter explained one important detail.

The truck was a hero.

That changed the conversation.

Larry Cullen had served as a U.S. Marine in Vietnam, and Peter regarded him as a genuine hero. Rather than encouraging his brother to create the sort of booming, exaggerated tough-guy performance that might have been expected from an animated military leader, Larry pushed him in the opposite direction.

Don’t play a fake Hollywood hero, he advised him.

A real hero should have compassion and dignity. He should be powerful without constantly needing to prove that power.

Most importantly, he should be strong enough to be gentle.

Peter Cullen took those words with him to the audition.

Then something remarkable happened.

Larry’s Voice Came Out of Peter

When Cullen entered the recording booth and finally saw the description of Optimus Prime, he immediately thought of his brother.

The connection went beyond simply making the robot sound deep or authoritative.

Cullen later explained that Larry had become the model for the entire character.

At BotCon in 2010, Cullen said his brother inspired both Optimus’ voice and personality. He associated Larry with qualities including honor, courage, dignity and integrity — exactly the characteristics Cullen believed the Autobot leader needed.

Years later, speaking to the Associated Press, Cullen described the process even more simply.

He said he had essentially performed an impression of Larry.

Every time Optimus spoke, Cullen could still hear his brother inside the character.

That helps explain something unusual about Optimus Prime.

For a character created partly to help sell transforming robot toys, he rarely sounded like a commercial superhero stereotype.

There was weight behind him.

Optimus could be intimidating, but he did not need to behave like a bully. He could command Autobots in battle while still showing compassion toward weaker characters. His authority seemed to come from responsibility rather than ego.

Those qualities were not accidental.

They came from the man Cullen was thinking about inside the booth.

The Role Became Bigger Than Anyone Expected

Cullen could not have known what would happen next.

The original Transformers cartoon became a defining piece of 1980s pop culture, and Optimus Prime became one of the franchise’s most recognizable characters.

The toys continued. New animated series followed. Comics, video games and movies expanded the mythology.

Then, more than two decades after Cullen first auditioned, Hollywood transformed Transformers into a major live-action film franchise.

When Michael Bay’s Transformers arrived in theaters in 2007, Cullen was once again the voice of Optimus Prime.

He continued through films including Revenge of the Fallen, Dark of the Moon, Age of Extinction, The Last Knight, Bumblebee and 2023’s Transformers: Rise of the Beasts.

The children who had first heard Cullen’s voice in the 1980s were now adults.

Some were bringing their own children to see Optimus Prime.

Cullen noticed.

He once described meeting fathers at conventions who became emotional when they heard him perform the voice. Often, he said, the parents seemed even more overwhelmed than their children.

What began as a single audition had become a connection stretching across generations.

Optimus Was Only Part of Cullen’s Extraordinary Career

Although Optimus Prime became Cullen’s signature role, his career extended far beyond Transformers.

He voiced Eeyore in numerous Winnie-the-Pooh projects, including Disney’s The New Adventures of Winnie the Pooh. He performed the voice of the villainous KARR in Knight Rider and contributed vocal work to the 1976 remake of King Kong.

In one of the strangest contrasts to Optimus Prime’s eloquent speeches, Cullen also created the distinctive clicking vocal effects associated with the alien hunter in 1987’s Predator.

His work eventually earned him a Lifetime Achievement Award at the Children’s & Family Emmy Awards in 2023.

Yet Optimus remained special.

Cullen said the character was his favorite partly because of the enormous effect it had on audiences.

And perhaps that emotional connection existed because Optimus was never merely a voice Cullen invented for a job.

He carried someone real inside it.

The Words Came Back One Final Time

After Cullen’s death, his family asked people honoring his legacy to serve their communities and lead with compassion, integrity and loyalty.

Then came a phrase Transformers fans immediately recognized from Cullen’s stories about his brother:

“Be strong enough to be gentle.”

It was essentially the lesson Larry had given Peter before the Optimus Prime audition.

More than 40 years earlier, one brother had told another what a real hero should be.

Peter Cullen carried that idea into a recording booth and turned it into one of the most recognizable characters in animation history.

Millions of children grew up thinking they were listening to a giant robot from another planet.

In a way, they were actually listening to Peter Cullen remember his brother.