CANNES – Something’s ailing Benicio Del Toro’s title character in “Jimmy P. (Psychotherapy of a Plains Indian),” but let’s lay that to one side for now. More pressingly, what is up with Arnaud Desplechin? The French writer-director is typically one of his country’s liveliest talents, with big, crowded, unapologetically chaotic films like “A Christmas Tale” and “Kings and Queen” bristling with emotional and intellectual curiosity — but he’s come a cropper in this lethargic, self-important psychiatry study, which he himself seems to have directed from the couch.

Playing to virtually none of his strengths either as a stylist and a storyteller, it’s a curious misfire from a director whom one had hoped would return stronger to English-language fare after 2000’s coolly received “Esther Kahn.” Certainly, neither its doughy structure nor its vague, tin-eared evocation of post-WWII middle America are a immediately indicative of a passion project that Desplechin has reportedly been nurturing for over two decades: we’re always plagued the longest by the problems we have the least natural ability to solve, and that’s a pearl of psychiatric wisdom you can have for free.

It’s not hard to see why Desplechin labored so long in bringing renowned Hungarian-born ethnologist and psychoanalyst George Devereux’s 1951 text “Reality and Dream” to the screen. The academic material doesn’t present a surfeit of natural options to dramatists, and despite the best efforts of the script — by Desplechin with Julie Peyr and American film critic Kent Jones — to make a character study of a case study, the onscreen incarnations of Devereux and one of his most intensely scrutinized patients, Native American war vet James Picard, aren’t given much life in the film outside their own restlessly talking heads. And while even the stuffiest psychiatry dramas (here’s looking at you, “A Dangerous Method”) can derive some voyeuristic tension from the testy push-pull of doctor-patient negotiation, “Jimmy P.” offers us two characters who cooperate to mutually beneficial effect pretty much from the get-go — which, while it presumably makes a rich and substantive case study in “Reality and Dream,” is a rather wan dynamic on which to build a feature film.

The film’s scene-setting is not unpromising: it’s 1948, and the brooding, taciturn Picard is being carted around Kansas by his no-nonsense sister (Michelle Thrush, all-too-briefly essaying the most convincing human being in the entire film) to various medical specialists, none of whom can adequately treat the mood swings and migraines that have plagued him since a wartime head injury. “He may be merely an Indian,” one doctor dares to offer as diagnosis, and it seems we may be in for a penetrating personal account from one of the most shameful collective social blind spots in American history.

Until we aren’t. Enter Mathieu Amalric’s bumbling, brilliant, rule-busting Devereux, a specialist in Mojave psychology, and all personal conflict in the film turns to mush; as written and played here, he’s effectively Patch Adams with an indeterminate accent and more impressive degree, and it isn’t long before his unorthodox methods are coaxing Catholic guilt and cathartic revelations of childhood sexual abuse out of Picard by the brainload.

Slowly but surely, his headaches disappear — only to be inherited by the viewer, in response to the film’s tortuous structure of dream-interrupted one-on-ones, and the duelling accent work in the leads’ monotonously mannered performances. (This is Amalric’s fifth collaboration with the director: one note they can take going forward is that the actor clearly does better with Desplechin characters who are more patient than shrink.) If it’s a relief when Gina McKee rather randomly turns up as Devereux’s fruity-toned mistress, that not because her wholly incidental character makes any sense whatsoever: she dispenses some snappy bon mots, rides a horse for a bit and leaves a farewell note bizarrely delivered as a seated to-camera address, but we’ll take the switches in rhythm we can get at this point.

The intrusion of McKee suggests even Desplechin would admit two-handers aren’t really his thing. His best film thrive on mess and bustle and fragmentation, and even the burnished, autumnal visual beauty of “Jimmy P.” — Stephane Fontaine’s serene, serge-textured cinematography is easily the film’s most accomplished feature — seems a strain on his sensibility. (Howard Shore’s ceaselessly swelling score, meanwhile, is simply a strain.) “Jimmy P.” culminates with its eponymous patient making a further, rather drastic bid for physical recovery: a complicated operation that involves an injection of pure oxygen into his spine. Here’s hoping Desplechin, with this airless film finally out of his system (and sure to be swiftly out of ours), has something similar in mind.