When KJ Apa was being considered to star as Jimmy Stewart in the new biopic “Jimmy,” he took another look at “It’s a Wonderful Life.”

“I was watching and thinking, how am I going to pull this off?” the actor, who is best known for his starring role as Archie in the CW coming-of-age series “Riverdale,” tells me. “I don’t look anything like him. I don’t sound anything like him. I’m not as tall as him. Like none of it lined up, but by the end of the movie, I was like, ‘I have to do it.’”

“Jimmy,” directed by Aaron Burns, chronicles Stewart’s rise in Hollywood and time fighting in WWII. After winning an Oscar for “The Philadelphia Story” in 1941, he was drafted to serve in the war. Rather than use his celebrity clout to dodge the draft, Stewart enlisted and flew 20 missions over Europe and rose to the rank of colonel.

When he returned home from the war, however, Hollywood seemed to have forgotten about him. Stewart considered moving back home to Indiana, Pennsylvania to work in his father’s general store. But Frank Capra convinced him to star as George Bailey in what is now considered one of the greatest films of all time, the 1946 Christmas classic “It’s a Wonderful Life.”

“Frank Capra gave him that script and he was going through a lot of what George Bailey was going through,” said Apa, who will receive a Spotlight Award on Oct. 18 at the Newport Beach Film Festival. “I honestly think ‘It’s a Wonderful Life’ was like therapy for him. He was able to continue in his career. His daughter Kelly told me when he came back from the war, he said he wasn’t sure if he was a retired actor or a retired pilot. He just didn’t know what he was doing. I think he was lost.”

I sat down with Apa before the recent world premiere of “Jimmy” at the Heartland Film Festival in Indianapolis. We talked about finding Stewart’s voice, being open about his addictions and recovery and speculation that he is the singer Mr. Fantasy.

Quick note: I make a brief cameo in the film as bandleader Glenn Miller.

What did you know about Jimmy Stewart before signing on for the movie?

Growing up in New Zealand, I didn’t know much about old Hollywood. I wasn’t familiar with the old Hollywood movie stars.

Take me through the process of getting Jimmy Stewart’s voice?

I was meant to shoot a movie called “The Sublime Biopic” about the band Sublime. [Apa was set to star as the band’s late frontman Bradley Nowell]. I was working with a dialect coach, Eliza Simpson, at the time and she’s phenomenal. I worked with her every second of every day for “Jimmy.” I watched all his films. I would listen to him over and over and over again. You just have to spend a lot of time listening and repeating, listening and repeating, listening and repeating.

His voice is so specific and iconic. It’s not like you could practice by using it when you’re at Starbucks.

For “Sublime,” I would do that. I’d go out and I’d talk to people in the accent. No one would think that wasn’t my voice. You’re totally right with this one. I was in no man’s land for the duration of the prep. I had not shared it with anyone. I was terrified. My only sounding board was Eliza.

What was it like shooting your first scene using Jimmy’s voice?

It was really nerve-wracking. It could go two ways. I realized it was such a fine line.

You didn’t want to sound like an “SNL” skit.

Exactly, which, by the way, I did read some of the things online. They were pretty funny. But at the end of the day, it’s completely out of my control. I can only focus on what I’m doing with the actors that I’m working with and with Aaron. There’s only so much that I can control. If I look like a fool, I look like a fool. I did my best.

What do you want people to learn about Jimmy Stewart from the movie?

I want people to have the same feeling I had when I first read the script, which was it was unimaginable for me to believe that he was such a highly decorated actor, and then served for over 20 years as a B-24 bomber pilot. He was involved with the D-Day invasion and then came back and did like 50-plus movies. Frank Capra gave him that script and he was going through a lot of what George Bailey was going through. I honestly think “It’s a Wonderful Life” was like therapy for him. He was able to continue in his career. His daughter Kelly told me when he came back from the war, he said he wasn’t sure if he was a retired actor or a retired pilot. He just didn’t know what he was doing. I think he was lost. But I want people to know what he did for this country.

Has Mr. Fantasy seen the film?

I have to ask him, but I think Mr. Fantasy would love this movie because he is such a positive guy and it’s a movie that will make you feel good. It’s not political. It’s not controversial. It’s about a man who doesn’t do the easy thing. He does the thing that he feels is the right thing to do, which is to serve his country because his father did the same, and his father’s father did the same.

What do you say when people ask you if you’re Mr. Fantasy?

I say the truth, which is no. Listen, I wish that I could have the confidence to do what that guy does. I mean, he’s just so himself. The energy that guy has!

This summer on Dylan Sprouse’s podcast “Wildmen,” you talked about your struggles with food. You’ve also been open about your addiction and recovery. Do you ever worry that you’re putting yourself out there too much?

It really doesn’t get more simple than that I’m a good friend of Dylan Sprouse. He asked me to come on his podcast. I had just dropped my son back off to his mum’s house. I was completely unprepared. I just rolled up and sat down, and we started talking, and I felt in that moment like, this is who I am. This is a huge part of who I am. These thoughts that go into an eating disorder or drug addiction or other addictions, they are all part of who I am and have taken up so much of my life. I’ve had so many people come up to me saying, “Thanks for sharing that.” I honestly didn’t realize that it would have that reaction. It was cool.

“Jimmy” is in theaters on Nov. 6.

This Q&A was edited for length and clarity.