It was Costin's first show as McQueen's art director at the Royal Horticultural Hall in London's Victoria last year that helped persuade Givenchy to enlist McQueen. Just as the Romans flooded their amphitheatres to stage sea battles, Costin turned the hall into a huge lake, three inches deep.
"I did tests on the floor to make sure the water wouldn't ripple as the audience came in. It appeared to be a mirror, but as the first model walked down the steps and touched the surface, the mirror shattered." In their perspex heels, the models looked as though they were walking on water.
If McQueen - who came out fighting with his bumster trousers and car- crash aesthetic - is dressing the headlong rush towards the end of the century, Costin is taking care of the interior design. "I'm there to present an environment which acts as a foil or a taster," he says. "It shouldn't overpower what they are about to see."
Last February, in the run-down surroundings of Borough Market, near London Bridge, Costin built a post-apocalypse urban scene for the "It's A Jungle Out There" show, in which the models came on like wild animals. Costin erected a 40ft screen of corrugated iron, drilled with "bullet" holes. A light behind and smoke in front pierced the air with tiny beams. Crashed Seventies Mercedes and Capris - "gangster cars" - had fire pots scattered round them, some of which were kicked over as punters who had been locked out stormed the barriers.
Costin, 33, began by playing with fire. He and his brother made horror films on Super-8 when Costin was about ten, and for one of them, Costin wrapped fireproof, ironing-board covers round his arms, set fire to them with lighter fuel and rushed towards the camera. Later, they interred one of their friends. "We waited till our parents were out and buried him in the garden with a hosepipe so he could breathe. We took the hosepipe away at the last moment then started filming as he pushed his way up."
It isn't difficult to see why McQueen and Costin - whose gentle features and mild manner give little indication of the strange world inside his head - were made for each other.
The son of antique dealers in Greenwich, Costin went to Wimbledon School of Art, partly because Derek Jarman, whose work he admired, was a part- time lecturer there. After graduating in 1984, he became Jarman's assistant, preparing canvases, mixing paints.
He made the jewellery for Jarman's film Caravaggio, and also had a bit part. "I was the court jeweller and I had my teeth gilded, but the smile looked a bit hammy, so I had to keep my mouth shut and look dour in bed."
After working with Jarman for a year and a half, he set up a studio squat in Bloomsbury with old college friends. Then, in 1991, set up another squatted space in Islington. His work veered between jewellery and body sculpture: "You couldn't necessarily wear it," he says. "It might rip you to shreds." One piece, a grim metal choker in his 1995 ICA show, "Snuffed", would indeed choke you if you were misguided enough to put it on.
Notoriety came when he exhibited a necklace from which hung phials of his own sperm. The gallery was raided on the show's opening day, although he says "the police came back the next day and apologised - a couple of people had complained so they'd had to investigate."
McQueen and Costin became friends in the late Eighties. McQueen admired Costin's body sculpture and in shows such as "The Hunger" and "Highland Rape" Costin designed necklaces and head-dresses for McQueen's angels with bruised faces. Then came the water catwalk, and McQueen's rapid upward trajectory.
Costin's own work has developed into mainly conceptual pieces and digitally manipulated photographs mounted on light boxes, such as "Senseless": five mounts, each depicting one sensory loss - Costin cutting out his tongue, slicing off his nose, gouging out his eyes. But he is spending more and more time with McQueen. Is it easy to work with the bad boy of British fashion? "Oh, totally. He knows I'm not going to go off at some bizarre tangent and I know what he expects."
Katy England, McQueen's assistant and muse, finds Costin easy to work with. "Sometimes, after a hard day in Paris, we'll say to him we need a bit of fiddling round with this bird's beak or a bit of jewellery and he'll do it," she says. "Even if you ask him to stick beads on or something, he will do it."
Costin isn't allowed to say much about the next show, at the end of September. "All I can tell you is that it will weigh 40 tons. It is a mammoth feat of engineering, and producing it is a real campaign." In the war against blandness, Costin is fighting on the front line.
Simon Costin's work can be seen in the show "Bad August", until 27 August, at Richard Salmon, 59 South Edwardes Square, London W8, 0171 602 9494.