In their new movie, "Ladder 49," John Travolta and Joaquin Phoenix play men who play with fire -- or, more accurately, fight it. And when we joined them on the set, we learned that shooting the action scenes often put them in harm's way.

"It's a scary moment when you come upon a wall of flame," Joaquin said.

"We're dealing with the real thing there," added John.

"Extra" went behind the scenes of the firefighter drama, for the stars's first day on the Burn Stage, a custom-built set that the moviemakers actually torch for each take.

"If anything happens, we have 'Extra' here -- we have it all on tape," John joked as he suited up.

The on-set fires -- which are real but regulated -- were way too fierce for us to follow the stars inside once shooting started. But there's no question that the intensity of the fires matches what real firefighters must deal with on a daily basis.

Joaquin Phoenix went out on a genuine call with a fire company to prepare for his role. He plays a trapped firefighter, flashing back to his early days on the job. Travolta plays his mentor, the seasoned veteran -- which is kind of appropriate. "I like being the fellow that has the experience and shares it with younger people," John told us.

Travolta and company will burn up the big screen this fall.