Why couldn’t “The Sessions” stay on the course it sets out on? John Hawkes gives a star-making performance as real-life writer Mark O’Brien, a man who contracted polio as a child and has been held immobile by weak muscles and an inability to breathe for too long without help from an iron lung. William H. Macy does terrific work as a priest who Mark confides in about his sexual awakening. This happens to coincide with an assignment given to him by his editor which involves delving in to how couples with disabilities handle sex. Mark is a virgin, one of several punishments he believes God is enacting on him. Mark is wrecked with shame, not only from his illness but from the amount of attention he required as a young child (his parents caring for him led to a tragedy in the family) and from a moral standpoint now (whenever someone bathes him, he is prone to ejaculation).

Early on you feel like writer-director Ben Lewin’s film will be more about the psychological and emotional toll sex can have on a guilty conscience. You can’t love another if you feel like you’re always at fault, something anyone who has suffered from a lifelong problem knows full well. And that’s where Helen Hunt’s sex specialist, Cheryl, comes in. Hunt’s performance is tender and understanding, putting Mark at ease.

Only where the movie goes from here is as unbelievable as it is unnecessary. Soon Mark is sending love poems and Cheryl seems to be considering leaving her husband (Adam Arkin) and Lewin does next to nothing to prove this relationship should exist anywhere other than professionally. The laughs come easily, sometimes too much so (Mark and the priest hold their sex talks right at the altar of the church so anyone looking to pray can hear Mark talk about “vagina”). Hawkes owns this movie, giving a performance that’s vulnerable, sweet, funny, and identifiable but “The Sessions” as a whole feels slight and unaffective, failing to capitalize on two interesting characters.