It's scarcely possible to type the title of this show – deep breath, let's get it over with, here we go, Rain Man – without feeling a slight twinge of worry that one will wind up on the wrong side of the law. There have been defamatory and wholly unsubstantiated rumours of sexual dalliance in a London hotel – although who allegedly did what to whom and how is unclear even at the level of question because of subsequent threats to the press from solicitors.
To whom is this of the remotest interest? Well, the producers of the show for one. As a clever, paradoxical twist to the publicity game, the situation could hardly be bettered. And of course, the devilish brilliance of those producers is evident in the majestic package we have here. In a profoundly original stroke, they have brought to the West End a stage adaptation of a famous movie (the one that starred Tom Cruise and Dustin Hoffman). And – this is where it becomes almost painfully subtle – they have placed at the helm of the project a Hollywood movie hunk, Joss Hartnett, with scant stage experience. This kind of thing could catch on.
When the curtain was 15 minutes late in rising, one began to wonder whether it was perhaps because backstage consultations with lawyers had overrun. But, no, it turns out that some of Mr Hartnett's young fans aren't aware of the theatre etiquette that requires you to be in your seat by a specified time.
I'm not knocking the desire to see a film star in the flesh. I'm just saying that you'd get a poor idea of the possibilities of theatre as an art form from his dull, undistinguished performance. It's his co-star, Adam Godley, who shows you what real acting is.
Godley plays Raymond Babbitt, the "developmentally disabled" and institutionalised autistic savant who can memorise the telephone directory but has no friends in it that he could ring up. Enter his long-lost brother, Charlie (Hartnett) who is peeved at their late father's will which leaves $12m to the crank and nothing to Charlie, the financially strapped car dealer.
Hartnett's inexperience is at its most damaging in the silences which punctuate and accelerate the tentative growing rapport between the siblings. These silences are nerveless absences of speech here rather than the charged moment of intensity that they should be. And when Hartnett speaks, he can't always wrap his mouth crisply enough round the fast-talking dialogue.
Godley, though, is an anguishing joy. His Raymond has all the premature elderliness of the permanently precocious. He looks like a mix of floppy toy monkey and a dogmatic stickler-for-routine old lady. He comes across as a being infinitely marooned in a vast loneliness that it would be harrowing to plumb. Ideally, you would want from Hartnett some suggestion that Charlie, though on the ball and in full use of his balls, has complementary emotional difficulties. That, though, would require a balance of talents less lopsided than those arranged for us by the esteemed producers.