Patrick Ryan

USA TODAY

PARK CITY, Utah — Jennifer Aniston is no G.I. Jane.

"I don't normally gravitate toward being in war films. I just find them hard to watch sometimes," says the actress, who made an exception for poetic Iraq War drama The Yellow Birds, which premiered at Sundance Film Festival Saturday. Adapted from Kevin Powers' best-selling 2012 novel, the script by David Lowery and R.F.I. Porto was "written so beautifully and in such a way I had never experienced. This was such a humanizing portrayal of these (soldiers) and the mothers back at home."

Birds — which is still seeking U.S. distribution — follows two young privates, Brandon Bartle (Alden Ehrenreich, aka young Han Solo) and Daniel "Murph" Murphy (Mud's Tye Sheridan), who become fast friends in boot camp and bond on the front lines. So when Murph goes missing in action overseas, Bartle is haunted by the promise he made to his comrade's mom, Maureen (Aniston), to keep her son safe.

Like her Screen Actors Guild Award-nominated turn in Cake in 2014, Aniston, 47, goes refreshingly vanity-free to play Maureen: sporting dowdy sweaters, poofy bangs and virtually no makeup. To tap into the mindset of her character — who faces off against government officials as she searches for answers about Murph's whereabouts — the Friends alum read articles and interviews with veterans' grieving parents, and worked extensively with her acting coach.

"I have obviously never lost a child or had a child that's gone off to war, so just trying to find something in my own world that resonates with that reality (was challenging)," Aniston says. "It's a pretty easy thing to connect to, just the excruciating pain of having something you love very dearly be taken" and the "frustration (when) no one listens. The (soldiers') parents are sitting there asking questions, but no one really knows what's going on over there."

Some of Birds' most stirring scenes are those between Aniston and Ehrenreich, 27, who hadn't yet been cast as the Star Wars spinoff's space cowboy when the film shot in Morocco in fall 2015.

"He hadn't yet gotten his fabulous Han Solo, but (working with him) was like watching a young Harrison Ford or Leo DiCaprio," Aniston says. "You name it — all the greats."