There are some bizarre items on the agenda of today's production meeting at York Theatre Royal. Topics include "dinosaur topiary" and Pontius Pilate's underwear, while the wardrobe supervisor is anxious to know God's measurements. "Ineffable and unknowable," someone suggests. "Very funny," comes the reply. "But I've got nearly 1,000 costumes to make and I need his inside leg."

Unfortunately, no one knows God's inside leg, or even, at this stage, who God is likely to be. There are, however, 500 amateur actors drawn from the local community who – along with 1,200 behind-the-scenes volunteers, a brass band and a 100-strong choir – have been working since Easter to prepare for this new production of the York Mystery Plays. Since the early 1950s, it has been traditional to perform these 14th-century religious dramas in the open air, among the ruins of St Mary's Abbey in York. Yet 12 years have elapsed since the cycle was last seen: a lavish staging by Gregory Doran presented in the nave of York Minster, marked the new millennium.

"There's a feeling within the city that the plays really belong outdoors," says Damian Cruden, artistic director of York Theatre Royal. "In its heyday, the abbey would have been awash with colour, with the story of creation spread over the walls and windows. The plays were aimed at ordinary folk who couldn't read the Bible – to give them a rollicking good time."

During the medieval period, biblical dramas were performed in towns up and down England: in cities craftsmen's guilds became responsible for performing specific stories (often taking the opportunity to do a bit of advertising; in York, the bakers did the story of the loaves and fishes, while nail-makers nabbed the crucifixion). Although fragments of all of them survive, York boasts one of the most complete cycles: 48 plays in all, acted in a day-long pageant dramatising the gospel from Genesis to the Last Judgment.

Given its age, the language is now difficult for non-experts, so a new version has been prepared by York-based playwright Mike Kenny, who has trimmed the work to a dozen or so episodes, and discretely modernised the text while retaining its robust, regional flavour. What were the challenges? "The first is deciding what to leave out – there's so much good stuff, it could easily go on for five hours." (He's right. I recall a bum-numbing production in the early 1990s, in which Robson Green's Christ didn't get crucified until nearly midnight.)

"The other problem," adds Cruden, "is dealing with an omnipotent central character, because there's no drama if the protagonist knows exactly what is going to happen. But God is a fallible workman who often gets things wrong. He builds the universe the hard way, often disappointed with the results."

Kenny and Cruden are the team behind the recent, acclaimed version of The Railway Children, staged with a real steam locomotive at York's National Railway Museum before transferring to Waterloo station in London. "Doing a show like that gave us the confidence to tackle something on the scale of the Mystery Plays," Cruden says.

The rehearsal I watch features several hundred participants milling round a sports hall. Some of the smaller angels scoot about on bicycles, while Cruden and co-director Paul Burbridge Burbridge appear to be directing in code. "Red 6 to F7," Cruden calls out. "Green 32 and 29 to J4." As there are two alternating casts of 250, the practical solution has been to divide the stage into a giant grid and issue each performer with a coloured T-shirt and a number. A volunteer stage manager has the near-impossible task of tracking these movements on a spreadsheet. "It's like a cosmological game of battleships," he sighs.

Modern revivals have traditionally boasted a professional actor as Christ. After I visit, it is decided that God and Jesus will be played by Ferdinand Kingsley, son of Ben. For the first time in many years, there will be a full-time devil as well: Graeme Hawley, who honed his satanic chops playing Coronation Street's notorious serial killer John Stape. Fitting in to such an enormous production isn't straightforward, but it helps that God and the Devil have become the best of friends. "That's actually the way Mike Kenny has written it," says Kingsley. "Lucifer is God's best mate, until he betrays him. It's the story of a failed relationship."

"It would be very boring to play Satan like some medieval pantomime villain," Hawley adds. "The beauty of the Mystery Plays is that they allow the Devil to develop into a complex character. He's really just a guy who finds himself with too much responsibility. It's his arrogance that becomes his undoing. I said to my wife, 'How do you go about playing the most arrogant being in the universe?' She said, 'I'm sure you'll manage somehow.'"

• This article was amended on 14 August 2012. The original said that for the first time, there will be a full-time devil as well as the actor playing Christ. This is not the case and has been corrected.