As Stevens explains, every couple of weeks he would have to go into a special booth, which he called the Tron cage, and would be sprayed in the face with 10,000 UV dots.
In case you’re wondering, yes, Stevens did have to do the famous waltz scene on stilts.
One wrong move and Emma Watson’s foot would be under steel, so Stevens had to become a master with his equipment.
Then whatever it was that he had been doing the last few weeks on set — be it sleeping, talking or roaring — he would have to do again in the Tron cage.