Jason Isbell Admits He Wondered 'What Am I Doing Here?' on “Killers of the Flower Moon ”Set“ ”(Exclusive)

The Grammy winner appears in the Academy Award-nominated film alongside Leonardo DiCaprio, Robert De Niro and Lily Gladstone

When he isn’t making music, Jason Isbell likes to watch strangers on the street trying to place him. “You can see the wheels turning,” the 45-year-old six-time Grammy winner tells PEOPLE in this week’s issue. “They’re thinking, ‘I just saw this dude’s face real close up, but I don’t know where'.”

Recently, the answer tends to involve his portrayal of Bill Smith — a former horse thief who marries into the Osage tribe — in the Oscar-nominated Killers of the Flower Moon, which he auditioned for and landed during the pandemic. Isbell admits he was “terrified” when filming began, as his biggest contributions to the industry prior had been soundtrack-related. (He wrote the song "Maybe It's Time" for 2018's A Star is Born, starring Bradley Cooper and Lady Gaga.) Sharing scenes with Leonardo DiCaprio and Robert De Niro left Isbell, at least temporarily, with a case of imposter syndrome.

“The first few days I was like, 'What am I doing here? This is ridiculous. I've never been in a movie before,'” Isbell recalls. “But when I stopped focusing on what could go wrong and thought about what had already gone right, that led me to the [conclusion] I was there for a reason. If you're too concerned about why you're there, then some part of you is questioning the vision and capabilities of the people working around you.”

Something that gave the “Cover Me Up” singer peace of mind was his director: legendary filmmaker Martin Scorsese. “I thought to myself, ‘Marty’s not going to let me ruin his $200 million picture. I’ve never watched one of his movies and thought, ‘That was great — except for that one hillbilly’,” Isbell jokes. “So I decided what I needed to do was my best and be grateful for it. Everything opened up from there.”

Isbell, who relishes the collaborative musical process with his band The 400 Unit, found himself admiring Scorsese's technique. "It was pretty heavy for me, watching the way Marty worked. I went into it thinking, 'Okay, this guy is the greatest director alive, so he's either going to be there micromanaging everything, or not there at all. But he finds the perfect medium. One night I made a suggestion for a shot, and he shot it. He's listening to people and taking ideas and everyone goes about their business and no one gets their feelings hurt."

Adds Isbell: "That was inspiring to me. I went back to Nashville and made a record [Weathervanes, Grammy winner for Best Americana Album] with the thought that you can keep your vision intact and still let everybody participate creatively in the process."

Related: Martin Scorsese Defends 3.5-Hour Runtime for Killers of the Flower Moon: 'Give Cinema Some Respect'

Collaborators and audiences alike agree that Isbell’s debut was a success. “Jason saw who Bill was, what he could have been capable of, and how to hide him in plain sight; with polite deflection and dandy piety,” Isbell’s Killers of the Flower Moon costar Lily Gladstone tells PEOPLE.

She adds, “Jason is an incredible judge of character, and his ability to translate that into performance and to articulate Bill with such clear layers in a relatively small amount of screen time was just delightful to watch.”

Related: Jason Isbell Speaks Out After Divorce from Amanda Shires: 'I'm in a Good Place' (Exclusive)

Gladstone, a Best Actress nominee, says she first knew Isbell from his music: "I first knew him from Drive-by Truckers [Isbell's previous band, which he left in 2007] ...so when I saw the casting announcement I dove back in [to his music]. 'Last of My Kind,' 'Letting You Go' and 'If We Were Vampires' all made me cry the first listen, and they're definitely in my regular rotation now."

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The affection is mutual. Says Isbell of Gladstone: "I've had friends who were on a rapid upward trajectory, but I've never seen [one like Gladstone's]. And she is the person most equipped to handle that kind of rocket to stardom. I don't think it's possible to change who Lily is. Whatever fame and celebrity comes her way, she's just going to use it to do cool stuff and help tell people's stories."

For more on Jason Isbell's life and career, pick up this week's issue of PEOPLE.

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