Only two kinds of actors are bad: those who can’t be themselves and those who you wish wouldn’t be. All the rest are likely to shine in movies by good directors, and most actors whose reputations aren’t illustriously artistic have mainly been unfortunate in their collaborations. Dwayne (The Rock) Johnson has long been in my private pantheon of actors who are awaiting their showcase, ever since his performance in Michael Bay’s “Pain & Gain,” from 2013. Since then, Johnson has piled hit atop hit without stretching his artistry; now, in the title role of “The Smashing Machine,” Benny Safdie’s bio-pic about the former mixed-martial-arts star Mark Kerr, Johnson does the substantial work of bringing a noteworthy character to life, by infusing the role with his own expansive personality—perhaps unsurprisingly, given his background as a professional wrestler. It’s a performance of flair and precision and imparts emotion to a script that lacks it.

“The Smashing Machine,” which Safdie both wrote and directed, portrays Mark (the character, as distinguished from the real-life Kerr) from the time of his first bout in the Ultimate Fighting Championship, in 1997, to 2000. The period begins with victories and growing fame—though his achievements are shadowed and threatened by substance-abuse issues and conflict with his girlfriend, Dawn Staples (Emily Blunt)—and peters out with his climactic defeat in a big-money tournament that owes its high financial stakes to his earlier success. Safdie’s approach to the story is divided against itself: as a writer, he takes an analytical view, creating scenes that clearly exemplify the fighter’s powers and troubles and the personality traits that contribute to both; but he films the story with loose documentary-like sequences and large-scale spectacle, neither of which matches the script’s expressive precision.

That’s where Johnson’s acting comes in. Paradoxically, his performance draws strength from the narrow focus of Safdie’s writing, which enables him to flesh out the specific trait, feeling, or impulse that each scene exemplifies, slotting them together like tiles in a mosaic to form Mark’s character. The most fascinating of these traits is shown early on, when a journalist interviews Mark after his first triumph. As Mark describes his strategies, his reflections quickly outleap the specifics of the sport and take on a philosophical dimension. He starts with the simple part: his need to “really assert” himself on his opponent. Then he frames victory in psychological terms, saying, “I’m going to physically impose my will onto” the person in the ring who has the temerity to fight him: “You really feel when that happens, when the person just lets go and totally withers away in your arms.” He describes inflicting pain in gory detail, and, as if hearing the implications of his own thoughts, says that everything “felt very evolutionary,” a sublimely sidelong word for actions and emotions that he recognizes as atavistic and feral.

Safdie’s presentation of these comments foregrounds Mark’s understanding of what’s profound, and profoundly disturbing, about mixed martial arts, and perhaps all martial arts—the channelling and professionalizing of brutality. The scene sets the tone for the entire film, but it’s a height that Safdie can’t sustain. He delivers a rational movie on an irrational subject, an externalized film on a story of subjective depths. Safdie is obviously fascinated by the fighter’s psychology, but he doesn’t successfully dramatize the extremity of Mark’s experiences—except in negative form, by way of his difficulty fitting into the norms of a noncombatant life style.

Mark lives with Dawn in bland comfort in suburban Phoenix, but there’s nothing bland about the way he makes his living, and the conditions that this imposes on his daily routine place enormous stress on the couple’s relationship; Dawn thinks of Mark’s athletic career as a job, whereas, for him, it’s a comprehensive way of life. He declares, before another match, that, to succeed, he has to remain “one-hundred-per-cent concentrated” or else his “emotions will be running around everywhere,” and some of the movie’s most striking moments—and some of Johnson’s most impressive acting—involve Mark’s ironclad concentration when events are spinning out of control. His domestic life entails set regimens, including precise dietary requirements, and when Dawn gets the ingredients of his morning shake wrong (skim milk instead of whole, half a banana instead of one and a half bananas), he doesn’t rant or even criticize, but coolly dumps it out and makes himself another. His eerie calm seems to irritate Dawn even more than a candidly emotional confrontation would have done.

Dawn (a grievously underwritten character whom Blunt conjures with sheer actorly energy) is devoted and sympathetic but out of touch with the absolute, quasi-monastic nature of Mark’s athletic calling. (The script never makes clear whether their relationship had been different prior to his fighting début.) She joins him at the gym and helps him stretch, but her inability to pierce the solitude at the core of his pursuit drives her to a reckless decision that proves consequential. It leads to the movie’s best dramatic scene, one that plays to Safdie’s rational strengths. When Mark is in Japan to make his début on a mixed-martial-arts circuit called Pride, Dawn flies over and surprises him there, showing up in his locker room at exactly the wrong moment, just as he’s preparing to enter the ring for a bout. He barely acknowledges her presence, putting himself into a tightly controlled state of pressurized ferocity so intensely focussed and isolated, detached from all other concerns and thoughts, that she asks him if he’s high. (In pressing him for attention, she merely breaks his trance and, as the movie makes clear, is partly to blame for his bad results.) Johnson’s performance here is transcendent in its simplicity; doing as little as possible by means of will power, displaying an intense remoteness and an active impassivity, he strikingly evokes an athlete’s competitive self-sublimation.