In the movie “Oldboy,” Josh Brolin’s chubby, alcoholic character is imprisoned early in the movie and remains there for 20 years.

He spends that lonely time getting sober, getting fit — and plotting his revenge.

But that’s 20 years in a film, not in Brolin’s real life.

As an actor, Brolin had to fluctuate his body weight by 60 pounds in two weeks to fit into the schedule of the film, released in 2013.

New Orleans fitness trainer and bodybuilder Aaron Williamson, the engineer behind Brolin’s transformation, sat in a coffee shop and pulled up pictures of the actor on his phone.

On one side of the screen stands a laughing, bloated and puffy Brolin; on the other a gaunt, muscular man flexes his upper body as a wispy fake beard floats down to his chest.

Williamson works as a trainer and nutritional expert for many of the actors, directors and producers who come to New Orleans for film work.

As Williamson’s reputation grows, so does his credit list, which now includes such films as “Django Unchained,” “12 Years a Slave” and “G.I. Joe: Retaliation.”

But for Williamson, 35, it’s been a long road — through a troubled adolescence, a stint in the Marines, PTSD, and even a period of homelessness.

Originally from Daytona Beach, Williamson was a troubled kid who found his way into Florida’s drug and alcohol culture. Without many options after he graduated from high school, he joined the Marine Corps.

The structure and discipline proved to be what he needed, and Williamson quickly became the team leader in his first unit. When he deployed to Okinawa, Japan, he found his love for the gym. There was nothing else to do.

“On base there, it became an addiction or motivation. I looked forward to it in the morning and at night. If I didn’t get my gym time in, I was pissed off and lost my sanity.”

Williamson was assigned to a section of the Marine Corps that serves at military funerals. Each soldier has to fulfill a height and strength requirement, as well as project an appropriate demeanor for the somber duty.

Later he became a personal security officer for Gen. Peter Pace — at the time the vice-chairman of the Joint Chiefs of Staff — and traveled to Iraq and Afghanistan.

“It was amazing being the little me from Daytona Beach,” he said. “Being the bad kid, now I’m at the White House with the president and the chairman of the Joint Chiefs of Staff. My Marine Corps career was amazing.”

But after he retired from the military, life got difficult.

Williamson didn’t see a lot of options beyond the military and took work as a contractor in Iraq in identity management and biometrics.

It was a tough period, he said, his voice trailing off.

“When you’re over there for an extended period of time, you come back a different person,” he said. “I don’t care what anyone says or what you do. It becomes a new reality and you have to adjust coming back to the States. You see people die. You’re involved in things … occasionally you have to fight to stay alive.”

When he was given a verbal promise for a job in New Orleans, he took a leap of faith and migrated to the Crescent City in 2009.

But the job fell through and he couldn’t find stable work. Williamson ran out of money, lost his house in Florida, declared bankruptcy and ended up living in his car.

Eventually, he signed on as a contractor at Marine Forces Northern Command, a homeland security agency based in New Orleans. He also began working as a personal trainer on the side.

That’s when he saw his passion and career become one.

“I left the Marine Corps contracting job, and then jumped full time into the training. Seven days a week, 12- 14-, 16-hour days. I would sometimes sleep at the gym. It was my intro into fitness.

“(I thought) ‘Maybe I can do this. Maybe this will work out.’ ”

The business struggled. The stress weighed heavily on the veteran.

Then actor Zac Efron walked into Williamson’s gym.

“He came into the gym … and saw my photo on the wall, which was a Marine-type of photo of me,” Williamson said. “In the movie ‘The Lucky One,’ he plays a Marine Iraq veteran, so at that time he needed someone to help him portray a Marine in as far as ‘how do Marines walk and talk.’ So he brought me to production.”

From there, things snowballed.

For a few months, Williamson worked with a director of “Treme” who encouraged him to move to Los Angeles, saying there would be more work there. But as he prepared for a move, Williamson got a call from Sylvester Stallone for the production of “GI Joe: Retaliation.” Efron returned, and “Transformers” star Josh Duhamel required some help.

Since then film work has been steady. He stays busy running Aaron Williamson Fitness, and now he’s breaking into acting.

“Having had PTSD, I did therapy with a couple different people and it didn’t work. It didn’t do anything. For me it was a waste of money,” he said.

“But getting into acting, finding the right acting coach, became my therapy. It actually helped me connect emotions and feel things that I hadn’t felt.”

In April, Williamson learned he’d earned a role in “Terminator: Genisys.”

“It’s beyond amazing. To be a young kid, sitting back with my family, watching the original ‘Terminator,’ and then to be inspired by Arnold Schwarzenegger with my fitness career, my bodybuilding passion, to be here in New Orleans filming, and having a role in a ‘Terminator’ movie — it’s an amazing, amazing feeling.

“I’m attached to this city pretty deeply. I’ll forever be indebted to this place.”