TWO years of planning come to fruition tonight when the York Millennium Mystery Plays open for a sold-out run at York Minster. Only for Shekhar Kapur's 1998 film Elizabeth has the interior of the Gothic Minster undergone such a transformation. Even then it required all the hi-tech magic of the Men In White Coats special effects studio - and no doubt a chunk of the historical drama's £18 million budget - to remove the organ pipes, give the building first-floor gallery seating and turn York Minster into Westminster Abbey for Elizabeth's coronation in 1558.

For the first production of the medieval Mystery Plays in the Minster, director Gregory Doran has had to call upon more traditional theatrical tools, albeit with the aid of a production team of premier-league talent: designer Robert Jones, sound designer John A Leonard, production manager Stephen Rebbeck and lighting designer Michael Gunning.

This is a working building, with no dressing rooms, or in-built storage space for props or lighting gantries. All these facilities have had to be created for a cast of 200 by the production team, either in the Minster or in Dean's Park.

The seating in the central Nave has been removed, replaced by 1,029 claret-covered seats in a tiered structure that rises to eight metres (25ft). The stage takes the form of giant steps, painted in tones to match the Minster stone, and it is a huge structure: 26 metres in length from front to back, 16 metres wide and over five metres from top to bottom.

From the back of the stage to the back row of the seating is 65 metres, and sound resonates for seven seconds in this cavernous building. That entails problems, problems, problems for the production team but what an exhilarating challenge too.

Robert Jones, who refers to tonight's opening performance as D-Day, has been "working properly" on the project for 18 months; in contrast, he has six months on average to put together a Royal Shakespeare Company production.

"In terms of time and commitment, it's a bit like working on an opera," he says. "When you first walk into the Minster and you see the sheer size of it, you think how on earth can we stage something in here? You can't fight or compete with the building in any way, so I've come up with a design that stands on its own yet works within the building.

"I felt very strongly from the start that the set should not look like a set but part of the Minster."

Chorus costume colours will match the stage design, while principals will be in vivid colours - burnt orange, deep blues and reds, to mirror the stained glass windows - in a cinematic device to make them stand out. "It will be like a camera focusing in on the leading man," says Robert.

As for scenery, there will be a minimum of clutter and no fuss. "We want to tell this big, bold story in as big and bold a way as possible. We have to apply the big, broad brushstroke, and not use minute detail which would get lost," he reasons.

"In a production of this scale it would be easy to swamp it with too many ideas. Instead it has to be strong, simple and speedy to convey all the information."

Work continues up to the last minute in readiness for tonight's opening. The steps were too yellow once the varnish was applied, so now they have been toned down, while sound engineer John A Leonard and his assistant John Owens are addressing the need to add two more closed-circuit television monitors to help the cast see musical director and conductor Richard Shephard in close-up.

John is candid about the building. "You have to be honest - acoustically the Minster is a nightmare. It's meant to be a cathedral not a performance space, and it's possibly one of the biggest spaces that anyone could work in. There's a very fine line between being intelligible and the audience not being able to understand a word that's being said, and we have to pinpoint that line," he says.

John Owens found the answer earlier this year: a 'localised audio' system made by the Dutch company Duran Audio for use in such difficult acoustic spaces as airports, railway stations and cathedrals.

Leonard and Owens then heard the system in a cathedral near Rotterdam, and now it will be making its British cathedral debut in the Mystery Plays.

Ironically, the Minster has recently installed 30 speakers but they were unsuitable for this production. Instead, the computerised Dutch system will be in use and it involves only one long, thin speaker, suitable for both the spoken word and Richard Shephard's musical score.

"It does it exactly what it says it does; it provides localised audio from a single source. It's a very clever device," says Leonard.

Production manager Stephen Rebbeck has overseen the Minster's transformation, from scaffolding to stone set. His duties even called on him to be the first person to try out the cross for the Crucifixion scene. "I have never done anything quite like this show before, and never anything as big as this before, putting it all together from scratch in a space that isn't a theatre," he says.

So speaks a man who recently knocked down half a building to create a theatre at the former Gainsborough Film Studios in London for Ralph Fiennes's performances of Richard II and Coriolanus.

York Millennium Mystery Plays, York Minster, tonight until July 22. Sold out, but standby tickets, if available, will be on sale from 6.45pm onwards in the South Transept.