That's right -- while it's possible to use CGI to make a building look like it's on fire, it's far more convincing to actually set it ablaze and yell "ACTION!" So the flames that were roaring in the background during the Basterds' murder spree were very much real, and those actors were actually inside -- that wasn't no goddamned green screen effect. The controlled blaze was supposed to never get closer than 20 feet from the actors, but fire rarely does exactly what it's told.
So, predictably, the flames roared across the theater almost immediately, and the fire was right on top of both actors within 30 seconds.
"Feel free to cut, Quentin. Whenever you're ready. Any time now."
Of course, that's the expensive set that's burning down around them, so this was their only chance to shoot the scene without rebuilding the whole damned thing. So everyone was forced to hurry and grab the shot before the out-of-control hellblaze overtook them.
The temperature of the fire quickly reached 2,000 degrees Fahrenheit and, while Tarantino and the crew were wearing fire suits, Roth and Doom were only coated in a jelly meant to protect their skin. The heat was so unbearable that Eli Roth passed out once the scene was over, and had to spend the next day with his head and hands in ice. But he still got off lucky.
The platform they were shooting on was about 10 seconds away from collapse when Tarantino called cut and everyone fled. Had they shot even a few seconds longer, Eli Roth and Omar Doom would have burned to death. Or almost to death -- it would have been awful either way.