CANNES – A Jim Jarmusch vampire movie? Sure, why not? Much of “Only Lovers Left Alive” seems to have been made in this spontaneous, scarcely thought-through spirit, which is responsible for what is both most appealing and most enervating about it. It’s a designer doodle of a dream, like much of Jarmusch’s work, though it’s clear some effort has gone into making it appear this cast-off. If the “Twilight” series has taught us anything, it’s that vampires are natural poseurs, which creates a stronger creative bond between Stephenie Meyer and the bequiffed crown prince of American indie cinema then you might have expected.

“Prince” might not be the appropriate word for an auteur now heading into his seventh decade. Indeed, Jarmusch’s youthful ennui and original-hipster styling have remained so consistent over the years that it’s fair to wonder if he’s something of a vampire himself. This may not even be the first vampire film he’s made on some level: think back to the languorous creatures of the dark that have populated such films as “Mystery Train” and “Night on Earth,” and a formal step into the genre seems positively overdue.

Not that Jarmusch does anything formally, of course. Adam and Eve, the disaffected pair of bloodsuckers at the center of this typically episodic narrative, may be immortal, but in every other respect they play very much by Jarmusch’s rules of characterization: they’re humanly world-weary, largely resistant to conflict and have impeccable taste in vinyl. (“Do you want to see the Motown Museum?” Adam Idly suggests on a nighttime drive. “I’m more of a Stax girl,” replies Eve.)

As played by the lank-haired, skinny-jeaned duo of Tom Hiddleston and Tilda Swinton — the former channelling any number of latter-day guitar-band frontmen, the latter chiefly channelling herself — they’re quite possibly the least alarming vampires ever put to screen, lightly canoodling, bickering over the vagaries of modern life, and scoring blood through non-invasive means wherever possible. (From the way they talk about it — at least until a final reel that introduces a faint element of peril to the wandering — blood is more marijuana than milk.)

“Only Lovers Left Alive” isn’t quite the desolate romantic vision suggested by its title, and the film itself is a little low on eroticism — even of the affected Jarmusch variety. It turns out the lovers spend significant lengths of time apart — breaks a couple can afford to take when neither party is getting any older — which may be why they interact more as loyal friends than passionate soul mates. The anaemic narrative hinges on the threat of Adam’s suicide; bored of his louche eternal life, he has recently bought a gun. Eve, haunted by premonitions of its use, travels from her Tangiers base to his ramshackle Detroit abode to talk him off the edge. And then some, and then some more.

It’s a thin premise for what amounts more to an extended sketch than a fully realized love story, though at least the one-ply joke is a droll one, played with good humor by the leads — particularly Swinton, who was pretty much born to deliver Jarmusch’s refined deadpan schtick. I’m not sure how many previous films have gotten comic mileage out of the possibility of modern-day Nosferati having known substantial historical figures, but it feels like more should have. According to Jarmusch, William Shakespeare and Christopher Marlowe were both vampires, and the latter (played by John Hurt in the agreeably befuddled manner of all his performances these days) lives on as Eva’s closest confidante. If the sheer listlessness of these characters and their travais proves swiftly irksome, that’s a familiar problem with Jarmusch — I suspect this is a fans-only effort, however en vogue the vampire genre may be these days.

The energy picks up for roughly a quarter-hour, when the ever-terrific Mia Wasikowska shows up as Eve’s selfish, shallow younger sister Ava. As she plays havoc with Adam’s temper and guitar collection alike, Ava’s presence fleetingly suggests this story may offer a broader view of vampire society, with its curious generational contrasts (relative to vampire years, of course) and varying degrees of human integration. But she departs all too quickly, and the air (or indeed the blood) goes out of the film when it returns to two-hander status, and its musings on mortality — or lack thereof — take on a self-serious, even sentimental, slant. “I’m barely still here,” whines Adam toward the end of this amusing but overlong amble, speaking for at least some drifting viewers. “We’re finished, aren’t we?”