James stars in Here Comes the Boom as a teacher who starts mixed-martial arts in order to save his school's music programme.
"It was a dream come true because I was fulfilling the dream of doing this and showing something different. It's not Paul Blart or goofy," he told Parade of the film.
James added: "It's funny but I wanted it to be real. We trained really hard, I changed my diet.
"I got professional trainers and fighters to work with me. It was gruelling but it was worth it."
James further explained that Here Comes the Boom was a deeply personal project for him because of his real-life support for school music programmes.
"I never played a musical instrument growing up but I knew kids who did and took it very seriously," the actor commented. "So we added that in the movie: how music affects these kids and can be used as a study aid.
"Music affects us all. We hear a song and it can take us back... These fighters... use it for inspiration or intimidation."
Here Comes the Boom - which also stars Salma Hayek and Bas Rutten - is now playing in the US and opens in the UK on November 9.