LOS ANGELES — Actors frequently give the reason for playing a character in a film as something like this: “It’s because it’s so different than anything I’ve ever done before!”

In the case of Ben Affleck and his portrayal of Christian Wolff, the title character in “The Accountant” (opening Friday), that actually is pretty much on target for the two-time Oscar winner.

Affleck plays a high-functioning autistic man and mathematics savant who also is a highly trained martial arts expert and deadly killer who cooks the books for some of the world’s most despicable mobsters and leaders of rogue states.

In a recent sit-down with Affleck and his “The Accountant” director Gavin O’Connor in L.A. recently, the actor and filmmaker said playing Wolff “was something like putting on an expressionless mask. It was a real challenge, but also an honor to to play him. I hadn’t read an autistic character in a film script like this. So often autistic people are seen as children. Here, I get to play a person with autism all grown up. We learn what happens to that little boy we see in flashbacks.

“Because it was such an opportunity to play something this fresh and new, I needed him to be as authentic as possible. So, to that end, I met with a lot of people and did a lot of research and read a lot of material so that I could deliver a portrayal that was as realistic and plausible as possible.”

Much of the film is set in the Chicago suburbs. Though tax breaks predicated the film be produced in Georgia, O’Connor (known for such films as “Warrior,” “Pride and Glory,” “Tumbleweeds” and “Miracle”) explained how he worked to replicate our suburban turf.

“We kept the story in the Chicago area because that’s the way it was in Bill Dubuque’s script, and we saw no reason to change it. Basically, because the story wasn’t set downtown, that was easier for us to reproduce in a believable way than if the movie was all about Chicago proper.

Turning back to his Wolff character, the current big-screen Batman said it “was nice to play a guy who isn’t a superhero. He’s capable of exceptional things — but within the realm of human possibility and human physicality. However, there’s a bittersweet note to it, as Christian’s character emerged from his father’s overly military way to prepare his sons from early childhood — including his older boy, who wasn’t autistic — to handle themselves in what he perceived as a scary and violent world. That’s what made it interesting for me.”

Asked if he now looked at his personal accountant a bit differently since making this movie, the actor quipped, “Are you kidding me?! My account is a killer! He kills my dreams! He’s always going, ‘No you can’t afford THAT! You can’t buy it! No! No! No!’

“I’ll ask, ‘Do I really have to pay that much in taxes?’ And that’s when he says, ‘Yes! Yes! Yes!’ ”

Often stars of films are allowed to keep favorite costumes or other accessories. That led to a suggestion that Affleck should have kept the tricked-out, fancy Airstream brand trailer that is seen in “The Accountant.” Affleck quickly noted, “I actually did buy that Airstream, obviously without the machine guns, the gold, the millions in cash, and the Jackson Pollock. But I did get to keep it. I got a deal from the production company, because, as you know, it was USED!”

“Listen, I knew how badly abused it was,” added Affleck, who explained, “So, now it’s my trailer on the set when I work on movies! Airstreams are smaller than the modern ones, but there’s something elegant about them.”

In the film, Affleck’s fellow Academy Award winner J.K Simmons plays a U.S. Treasury agent on the hunt to find Wolff.

For the character of Marybeth Medina, an analyst helping the agent, Simmons pointed out that he had a very serious conversation with the director about casting that role. “That was one of the first things about which Gavin and I spoke, after I made it clear I absolutely wanted to work with him and Ben and this terrific script.

“The actress who would play Marybeth Medina had not been hired. I told Gavin, ‘Please tell me you’re not going to get some gorgeous, glamorous young Hollywood actress to play Mary Beth Medina, because she’s got to have this kind of gravitas and intelligence and this toughness — all those aspects of this character.

“Gavin, nodded vigorously and said, ‘Absolutely! Absolutely!’

“Then he sent me pictures of Cynthia [Addai-Robinson, who plays a recurring role as Dr. Vicki Glass on the Chicago-made ‘Chicago Med’] and said, ‘This is the woman that I’m hiring.’ … I called him immediately, and said, ‘Dude! This is the opposite of what you said! She’s beautiful!’ … Gavin quietly told me, ‘Trust me. She’s going to be great.’ And he was right. She really delivered,” said Simmons with a happy nod.