LOS ANGELES—Producers lose interest in movies all the time. If you’re already working on four TV shows, it certainly makes sense to move on. And if a project keeps failing to find studio support, there would be doubts about its box-office potential, no matter how interesting the subject matter might be.

But Mark Wahlberg couldn’t quit on The Fighter. The true story about a Boston-area boxer who grew up in a family of nine children and fought the temptations of street life was very familiar. It was a story that Wahlberg had lived himself. He believed in the project, but after four years of working on it he became less inspired. He had an excuse. During those four years, he had starred in several movies and had become increasingly busy on the television front, putting in overtime as executive producer of Entourage, How to Make It in America, and In Treatment while working with Martin Scorsese to get Boardwalk Empire ready to launch on HBO.

The Fighter, which opens on December 17, stars Wahlberg as boxer Micky Ward, whose career has been spent in the shadow of his boxer half brother, Dicky Eklund (Christian Bale), once the pride of their hometown of Lowell, Massachusetts. But Dicky has become a crack addict, and Micky is a lovable loser who gets paid to make other fighters look good. It’s only when he drops his mother and brother from his corner that he begins to get respect from his peers.

In an L.A. hotel room, Wahlberg recalls that the best thing about the four-year wait was that he never stopped training. As a result, when the movie was finally ready to be made, he felt in good enough shape to take on the role of a boxer.

“The movie was a go, and then it fell apart, and I continued to train,” Wahlberg explains. “So after almost four years of training, I felt extremely confident to go in there and be believable as a boxer who was contending for the welterweight title. If someone had said at the beginning, ”˜You have to train four-and-a-half years to make this movie,’ I would have said, ”˜Absolutely not.’ But I felt that if I stopped training, I would be giving up on the movie. It was difficult some days getting out of bed, particularly when I was trying to make other films, but it was certainly worth it.”

Also inspiring him was the idea that the movie mirrored his own life. He says he could relate to much of Micky’s life and that he wanted to show that no matter how tough the breaks might be, success is always possible.

“I was in a lot of trouble, and I turned my life around. I think it makes such a good comparison to Micky’s journey. We both had nine kids in our family and grew up 30 minutes away from each other, so it seemed like it was a story you should try to get told.”