The role of the fighter, as conceived five years ago by the screenwriter Kurt Sutter (“Sons of Anarchy”) and the director Antoine Fuqua (“Training Day”), was originally meant for Eminem, who has only cameoed as himself in comedies since starring in the semi-autobiographical “8 Mile” more than a decade ago. But when it came time to commit to the film, he chose to finish and promote his album instead.

Still hoping to be involved with “Southpaw,” Eminem corralled some longtime friends and collaborators — 50 Cent, Slaughterhouse, Busta Rhymes — for an assault of guitar-heavy, underdog pump-up songs, including “Phenomenal,” his own montage-driving single. (In support of the project, which will be released on Shady Records the same day as the film, he has also been featured as a guest on Apple’s new “Beats 1” radio and showed up to sing Bob Seger songs with Stephen Colbert on a Michigan public access station.)

Conveniently, the “Southpaw” comeback narrative allowed Eminem to address some of his own anxieties about life as an aging rapper with an uneven latter career — “You’re moving onto the next, but is the respect gone?” he raps on “Kings Never Die.”

In conversation, Eminem was focused to the point of seeming solemn but also uncertain about his future, oscillating between intense eye contact and staring at the floor. He insisted not much had changed since his days as a hip-hop insurgent, including his sense of humor and competitive spirit. But he acknowledged that stasis, too, could be dangerous. These are excerpts from the conversation.

Q. How did you approach putting together a soundtrack for a film that wasn’t your own?

A. We wanted to make sure that the songs would remind you of the movie years down the road. When you think of “Don’t You (Forget About Me),” you think of “The Breakfast Club.” When you think of “Eye of the Tiger,” you think of Rocky.