Scott Glenn was so charmed by his “Eugene the Marine” co-star Jim Gaffigan that while filming a particularly wild fight scene, the 85-year-old modified a stunt so he didn’t accidentally kill the comedian.

“All that I was concerned with when we got to the end of this scene was making it physically as real as possible, but also as safe as possible,” Glenn says, during a Zoom conversation with Gaffigan for Variety. “For instance, I don’t know if Jim knows this, but at one point I put something called a ‘naked choke’ on him. I always made sure that my bicep was not really next to his carotid artery, because if you squeeze that thing wrong, you can very quickly halt blood going to the brain and hurt somebody permanently.”

For Glenn, it was just another day at the office — “You think about things like that,” he deadpans, as Gaffigan laughs, fully understanding that his life was literally in his co-star’s hands.

Glenn is booked and busy these days, with roles on the Apple TV+ crime comedy “Bad Monkey” and a part in the upcoming season of “The White Lotus.” Yet “Eugene the Marine,” an action-thriller co-written and directed by Hank Bedford, shows Glenn in a new light, one that harkens back to his early years, three of which were spent in the United States Marine Corps.

In the film, Glenn stars as Gene Lee Grady, a lonely military man mourning his late wife and living a quiet existence in his tidy house. Yet as he starts to open his life up to the people he interacts with each day — including a new friend (Shioli Kutsuna), a potential love interest (Annette O’Toole) and a sleazy local realtor (Gaffigan) — dark, violent secrets from the past appear. More details would ruin the clever and shocking twists, but Glenn is in full reluctant hero mode for much of the film.

Far before impactful roles in classic films like 1975’s “Nashville,” 1979’s “Apocalypse Now,” 1990’s “The Hunt for Red October” and 1991’s “The Silence of the Lambs,” Glenn was just a babyfaced grunt in the service — a fact Bedford didn’t realize before offering the actor the lead role.

“I talked to Hank and realized that he was going to let me do pretty much anything I wanted to with the character,” Glenn says. “He had a unique, painterly, unpredictable point of view of this whole thing, so I just loved talking to him and I decided to do it.

“I remember some days walking onto the set — one of the parts of the set decoration is my life,” he continues. “[My real-life wife] Carol and I are the wedding pictures. But there’s one shot of me that I think was taken after I got out of Parris Island, where every Marine goes through something called IT, or infantry training regiment. I’d look at that thing in the morning thinking, ‘I look like I’m 12 years old in the picture.’ But if someone had told me when that picture was taken it was going to be set decoration on a movie — forget about playing the lead, but that I was going to be acting at all, I would tell them they were out of their fucking mind. So much of my life feels like it sort of looped back on itself doing this.”

Glenn’s road to acting is unconventional, as he first became a reporter out of the Marines. He wanted to make it as a writer, but couldn’t hack the dialogue, so he took acting classes to help him be more naturalistic. Yet he soon started booking theatrical productions, television spots and, by 1970, films, and he was on his way to a sturdy career.

“I’m still learning about how to be an actor,” Glenn says. “But one of the things I realized, at least from my point of view, what gives a performance its juice, its magic — whether it’s comedic or serious — is its degree of spontaneity. The more you can be really in the now, the more electricity in what you’re doing.”

Bedford says that despite Glenn’s Hollywood history, he was a model collaborator in bringing Gene to life.

“As an actor, he brought so much to it as far as ideas for the script in a very respectful way, in a very collaborative way,” Bedford says. “Just wanting to help. He asked me very early on about these thoughts he had: ‘I hesitate to bring these things up.’ I said, ‘You’re making the movie better, and that’s it.’”

There was also an unspoken understanding between the two men to put their all into the production.

“With Scott, I have to be honest with you,” Bedford says. “I don’t know how he would feel about this, but he’s 85 and it was recognized by me and by him, although unsaid, that it could be his last movie. So he was bringing it. We were working long hours and he was just down.”

O’Toole, a Hollywood veteran in her own right who appeared in 1982’s “48 Hrs.,” 1983’s “Superman III” and the Netflix drama series “Virgin River,” says she was thrilled to collaborate with Glenn, saying, “I so wanted to work with him all my life.”

“[The chemistry working with Glenn] is one of those things that just occurs,” she says. “I don’t remember ever having that kind of rapport so quickly with another actor. It was like I’ve known him all my life as a person and as an actor, kind of like these two jigsaw puzzle pieces that are put together.”

Much of Glenn’s electricity comes through leading a varied cast, as Gene interacts with many different people as he branches out in life, including much younger new friends, his middle-aged son and an age-appropriate love interest. Gaffigan says he was thrilled that the film mixed it up in terms of its players.

“Obviously it’s a great thriller and there’s a lot of different genre elements, but I thought it was a really interesting take on these different generations of Americans and perspective,” he says. “Their flaws are possibly their strengths, and that doesn’t reflect on my character necessarily, but I really enjoyed that glimpse at the different generational elements, from the millennials to the baby boomers. It was an interesting stew.”

Glenn agrees, noting that he’s lucky to find roles that push him as an actor.

“The last five parts I’ve been offered, three of them I had a walker,” he says. “Three of them, I was dying of Alzheimer’s. It’s almost predictable — you get sort of the apologetic way you’re offered of these things: ‘He’s lost a step. He’s using a cane, but he’s still sharp.’ I’m thinking, ‘I’m 85, I still work out, hike mountains, do ridiculous amounts of pushups and kettlebell swings.’ What I see with ageism is just a conventional way of telling stories. It’s hard to tell whether you’re being overly sensitive or not about all this stuff.”

Yet films like “Eugene the Marine” keep Glenn focused on the present and looking forward to the future.

“My hope is that in the great unknown of my future, there’s a part I haven’t even thought about that’s going to be great,” he says. “My sense is that whatever really good and whatever really bad awaits me in the future is something I have no idea about right now.”

“Eugene the Marine” is currently seeking distribution.