But, wherever her talent comes from, Vincent plays dead for two hours better than a Rada-trained doornail. Her inertness contributes a lot to the success of Smith's first play, which won the Warehouse's 1996 International Playwriting Festival.
In Smith's twisted world, it's New Year's eve 1979, and Janet is dead before the play begins. It's hard to say which is more unappealing, the sight of this corpulent middle-aged woman decaying on the floor of her council flat, or the flat itself. With its beige walls enlivened by two pictures of cats and some flying geese, Anthony Lamble's set is a museum of bad taste.
Rifling through Janet's belongings are two men who look like police, but turn out to be GLC caretakers. Tom (Carl Davies), a dour Welshman in the Windsor Davies mode, and Porky (Roger Frost), a lugubrious Englishman with a face that's been wrung through a mangle, a skinhead daughter and the strange habit of wearing gloves indoors.
The two men's pilfering is soon interrupted by Janet's neighbour Norma (Susan Porrett), a drink-sodden sentimentalist, whose entrance is the catalyst for a macabre New Year's gathering.
Like Joe Orton, Smith likes to laugh in the face of death, and he can spot the humorous potential in suburban banality ("I always think," Norma muses, "the most unpleasant way to go would be to drown in an enormous vat of shampoo"). A more direct influence, perhaps, is the British sitcom, and in particular One Foot in the Grave.
Victor Meldrew would understand the irony of Porky's tale about a mother who is burnt to death while lighting a cigarette from the gas stove because her health-conscious daughter has confiscated her cigarette lighter. The death of a pet (Janet's cat, Mr Callaghan, who makes an appearance stiff with rigor mortis) is almost a weekly occurrence in Meldrewland.
With no broadcasting standards watchdogs to worry about, however, Smith is able to push back the boundaries of taste. A channel that broadcasts Animal Hospital would never countenance Norma's lengthy account of a cat being kicked to death by skinheads, a tale made horribly funny by her attempt to put a moral gloss on the salaciousness of gossip. An inspired pastiche of an erotic novel wouldn't have got past television's censors, less still a sex scene that employs the corpse's pneumatic stomach as a pillow.
There are times when the play seems to be trying to say something about the change in British culture that took place at the end of the Seventies. And, in its final moments, it attempts to reinvent itself as a tragedy. Fat Janet Is Dead doesn't quite work on either of these levels, but Smith's script often left me crying with laughter, and it is showcased by Jessica Dromgoole's well-paced production and a cast who (Janet excepted) display impeccable comic timing.
The Cracked Comic (New End), Stewart Permutt's new play about a heroin addict who becomes an overnight success as a stand-up comedian by telling sexist and racist jokes, boasts Kevin Day as its "comedy consultant". Yet, beyond observing that comedy promoters are ruthless in the pursuit of the next big thing, the play doesn't seem to offer much insight into the stand-up circuit. It's let down by a clutch of improbabilities.
"You could be a modern-day Bernard Manning," Gary Bleasdale's beefy thug of a promoter tells his new talent Albert (Peter Lee-Wilson), ignoring the fact that the olden-day one is still with us. Why would Channel 4 be interested in an act whose humour would be neutered by the requirements of the small screen (those TV watchdogs again)?
Anyway, the idea of the young Southern "politically incorrect" comedian has already been done: he's called Gerry Sadowitz, and he was last sighted doing his magic act on the London Fringe.
'Fat Janet Is Dead' is at the Warehouse, Croydon (0181-681 1257) to 15 June. 'The Cracked Comic' is at the New End, Hampstead (0171-794 0022) to SunReuse content