The last time I saw Carl Barat, he was still playing a rock star. Dirty Pretty Things’ final gig brought down the curtain on a part he played exceptionally well. A year on from their demise, out in the wilds of west London, Neil Sheppeck’s production of Sam Shepherd’s Fool for Love sees Barat audition for a different role. He’s always been a performer, with the Libertines’ and DPT’s gang mentality a fairly transparent protection against chronic insecurity and fear of isolation. There’s a similar protection afforded by having a part to play – a costume to wear and a script to follow which relieves the worry about being judged on your own merits. Doing so for a living seems a logical if precarious next step.

Barat’s casting was justified by the play’s director as offering an opportunity to broaden the appeal of theatre. Despite the presence tonight of a few boys with over-emphatic angular fringes, how much of this appeal is predicated on a preexisting fanbase is unclear. Barat’s co-star is Sadie Frost, who brings to the play an off-putting association with a certain clump of pseudo-bohemian post-Britpop layabouts who’ve spent the past decade in complacent self-congratulation and mutual assurance of their ‘edginess’, and by whom Barat is too interesting to get dragged down. Her crimes too petty and her position too peripheral for her to be an object of more than mild and dismissive irritation, Frost nevertheless isn’t that bad an actress, and it’s hardly her fault that society is still so steeped in the male gaze that a national newspaper reviewer can focus on her ‘good deal of thigh and cleavage’ as compensation for the rest of a dire first night (maybe for you it was, bucko; if I want thigh and cleavage I’ve got my own).

Carl knows a fair bit about being traded on your looks, of course. The man has been subject to a level of objectification that would have caused outrage if directed at a female performer. Being pimped by Pete Doherty as his ‘most photogenic of Libertines’ and by Alan McGee as his ‘sex symbol’ were top-down cues that hampered any degree of self-determination, and subsequent short-sighted slavering by hacks and hagiographers alike dripped a thick obscuring layer over Carl’s more interesting qualities. I’m hoping he can draw on some of the latter in his stage career: the rage and disgust that drives ‘Buzzards and Crows’; the grim dignity of ‘One to my Left’; the languid loucheness that he drapes around Client’s ‘Pornography’, the snarling aggression with which he throws himself into his live performances and the swaggering desperation with which he drinks. Consequently, seeing him tonight is a bit like going along to the Christmas-play performance of a favourite underachieving godson, willing him to do well.

So aside from this mixture of morbid curiosity and its weak, hopeful flipside, I’m not sure why I’m here tonight. Aside from the lack of things to do in Hammersmith on a wet Wednesday evening, I’m equally unsure why anyone else in the audience is here. And, for the opening scene and a good ten minutes after it, I’m not entirely sure why the actors are here either. When we enter, the dynamic duo are already onstage, Barat offering a striking slouched silhouette in ripped denim and Frost sprawled face-down on the bed. The Riverside Studio’s titchy set depicts a seedy motel room, shimmering in desert heat, a setting that mirrors the torrid and tawdry story of the two leads which jerkily unravels over the next hour. It is exasperating to observe that, while any glance at Carl’s career on and offstage will confirm that he’s more than capable both of exuding Brando-esque sexual menace and pulling off an accent, these qualities are only sporadically in evidence tonight. He starts the play peculiarly stiff-legged and twitchy, he and Frost edging awkwardly around each other with little sign of any sexual spark. The dialogue demands viciousness and bite, but it’s as if they know each other too well offstage to be anything other than scrupulously polite when on it.

It’s not until Carl’s character sinks into a chair and starts slugging shots of tequila that the pace picks up. Raising recollections of his past interactions with fanboys, he seems more at ease manhandling Neil Sheppeck as his hapless love rival than grappling with Frost. (Sheppeck’s not bad, by the way. More than not-bad is Gerard McDermott as Barat and Frost’s absent father: out of all the cast, he nails the accent with molasses-thick accuracy, even if at times I’m reminded of the opening narration from The Big Lebowski.) It’s refreshing to see Carl usefully employ a sense of comic timing, and get the laughs for doing so, when for years he’s been regarded as an object of inadvertent amusement. The climactic confrontation with the ghost of his father, where he raises himself from the floor with his whole frame hanging off a blazing deadlocked gaze, is a moment that’s worth the price of admission. It also occurs to me that one of this role’s major challenges for Carl must be the requirement to make it through an hour without a cigarette, which he achieves admirably. For all the ropey accents and the rudimentary rope-skills, I don’t leave disappointed.

Not that it matters, of course. Frost will carry on living the Primrose Hill lush life and Barat will continue to be criminally underrated, a fact that makes performances like this so uniquely infuriating. It’s a waste. This production is convincing raw material, but to fashion something great from it the performers need to throw themselves into the script’s verbal violence rather than shuffling so fastidiously around it. Carl is still hamstrung by inhibition and the volume needs turning up to 11. On tonight’s evidence Carl remains as frustrating an actor as he is a musician: perfectly adequate with flashes of brilliance when you wish he had the self-belief to be extraordinary.

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Fool for Love is in rep at the Riverside Studios until 20 March. Box Office: 020-8237 1111