Sebastian Stans new war drama film The Last Full Measure took 20 years to make.

The 37-year-old actor — best known for his role as Bucky Barnes in the Marvel Cinematic Universe — explained to uInterview exclusively how the process of making the film unfolded. The Last Full Measure is about real-life Vietnam War hero William “Pits” Pitsenbarger who tragically died at the age of 21 during a rescue mission in 1966 after saving countless lives during the war. More than 30 years later, Pentagon investigator Scott Huffman — played by Stan — probes a Medal of Honor request for Pits made by his friend and his parents.

“He’s trying to please his relationships and goes with [the story],” Stan explained of his character before adding that his character is loosely based on several actual people. “The film explores the selfless acts of a real-life hero that went unnoticed and why he might have gone unnoticed.”

Stan went on to say the movie — which was written and directed by Todd Robinson — tackles the subject of what selflessness and bravery truly mean and whether these are traits human beings are born with or learn to develop.

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Stan also raved about the opportunity to work alongside acting legends like Christopher Plummer, Ed Harris, William Hurt, Samuel L. Jackson, Diane Ladd and the late Peter Fonda.

“Like my character in the movie, I felt like I was down on this journey meeting all these guys,” said Stan. “There was an element of learning about acting, of being on my toes and also learning about life, myself and being a man.”

“The growth that comes from [working on] something like that is unmatched,” he added.

The Last Full Measure opened on limited release in the U.S. in October and will be released nationwide on Jan. 24.