EXCLUSIVE: Watch Shailene Woodley & Theo James Tackle 'The Wall' in 'Allegiant'

ET went behind the scenes of one of next year's most highly anticipated movies, The Divergent Series: Allegiant.

Shailene Woodley and Theo James return for the third installment in the Divergent series, and they filled ET in on what it was like filming one of the movie's most intense stunts -- tackling "The Wall."

In the sci-fi adventure flick, Beatrice Prior (Woodley) and Tobias Eaton (James) get past the wall enclosing Chicago and venture into the world outside, which they quickly learn is more dangerous than ever before. The duo must determine who they can trust as a ruthless battle erupts in their new environment, which threatens all humanity.

"We did this bit when Shai had to be facing down and I had to be the other way, and it was a little daunting," James said of scaling the 60-foot concrete structure.

WATCH: Kate Winslet Brings a Chill to 'Divergent'

"I was facing the ground, walking backwards," Woodley explained. "I was holding onto Theo's shoulder like it was the last thing!"

ET first met the stars of the film in 2013 and in two years the hit franchise has dramatically changed Miles Teller's life -- the Whiplash star is now practically a household name.

But one thing that hasn't changed is Teller's humility. He refused to accept the label of "heartthrob."

WATCH: Shailene & Theo Lock Lips in 'Divergent'

"Stop right there," he playfully told ET's Nischelle Turner. "I don't think I've ever picked a movie and been like, 'I'm going to look so sexy in this.'"

It might be hard to think of yourself as sexy when you're working on location in 90-degree Atlanta heat, as star Maggie Q explained.

WATCH: Shailene Reveals Brutal 'Divergent' Fight Scenes

"Your makeup just drips off your face," she said. "None of us are going to look even remotely good in this movie."

The Divergent Series: Allegiant hits theaters March 18, 2016.