In the tradition of “My Left Foot” and “The Diving Bell and the Butterfly,” Ben Lewin’s “The Sessions” (formerly titled “The Surrogate”) is a fact-based story about a physically challenged person that avoids mawkishness and actually manages to inspire and amuse. Based on an article by the late writer Mark O’Brien, whom a childhood bout of polio left paralyzed from the waist down and unable to breathe for long without artificial respiration, the film is also unusual in concentrating on the sex drive of its subject, who’s the sort of character often portrayed as a sort of plaster saint devoid of any such desires.

Simply put, O’Brien is depicted as a man with a keen sense of humor despite his condition, and with an equally keen interest in losing his virginity despite his disability. O’Brien had beaten the odds before—earning a college degree by going about campus on a motorized gurney that earned him attention from human-interest reporters. But his initial efforts at romance—directed toward his lovely nurse Amanda (Annika Marks)—miscarry. A counselor advises him to consider working with a sex therapist specializing in assisting patients to achieve intimacy with others. And his pastor, a committed but liberal priest, encourages him to try as well, despite his theological reservations.

One can imagine this story going wrong in any variety of ways. That it doesn’t is largely the result of the cheeky humor with which O’Brien told it and the sensitivity that Lewin brings to his adaptation. But it also depends to a great extent on the performances. As O’Brien John Hawkes is outstanding in an intensely difficult role, expressing a wide range of emotion despite the physical limitations of the part. He’s ably abetted by Helen Hunt as Cheryl, the wife and mother whose work is made easier by Josh (Adam Arkin), her writer husband, who gives her his support, up to as point. (The suggestion of romance that enters into the relationship between her and Mark—which causes her husband to grow jealous—is the one element of the film that doesn’t entirely convince, simply because Hunt plays it so gently. But to do otherwise would have pushed the film toward melodrama, and that would have been a mistake.) William H. Macy shrewdly limns the sort of priest who expressed the spirit of the Vatican II Council rather than the letter of church law. And the rest of the cast—Marks, Moon Bloodgood, W. Earl Brown—all offer winningly natural turns.

“The Sessions” doesn’t offer O’Brien’s complete story; for that, there’s a documentary called “Breathing Lessons” one might watch as a supplement. But through a combination of Hawkes’ remarkable performance and Lewin’s easygoing approach, which avoids solemnity in favor of a mood that’s more often lighthearted than ponderous (while managing to incorporate a good representation of O’Brien’s poetry), it captures the essence of his personality in a fashion that’s more revealing than a chronologically fuller portrait might have done. From the technical perspective this isn’t an overtly polished piece, but Geoffrey Simpson’s cinematography, though mostly straightforward, brings appropriately subtle shadings to the fairly explicit bedroom scenes (which are also infused with some deftly comic touches, especially when they’re conducted in a motel overseen by an inquisitive clerk, played by Ming Lo), while Lisa Bromwell’s crisp editing keeps things from flagging. And Marco Beltrami’s score is more delicate than his usual work.

The result is that Lewin, Hawkes and company have turned what on the surface sounds as though it might be an uncomfortable, titillating story into a sharply written, skillfully executed adult crowd-pleaser.