Scott Foley’s approach to acting starts the same way no matter the project. He believes it is his job to tell the most complete story possible through his performance. As in the case of the Investigation Discovery film “Final Vision,” which airs at 10 p.m. Sunday, Foley finds the script has most of what he needs but he continues to dig just a little deeper. 

What he found with the made-for-cable film is a script based on the true story of the longest-running criminal case in U.S. history. Jeffrey MacDonald (Foley) is a handsome, Ivy League-educated U.S. Army Green Beret doctor who was convicted of brutally murdering his pregnant wife and two young daughters in 1970. The story is told through the eyes of best-selling author Joe McGinniss (Dave Annable), who was approached by MacDonald to write a book about his personal nightmare as he was about to go to trial.

McGinniss originally believes he is writing about an innocent man being railroaded by a flawed legal system but eventually sees the twisted psychological at the core of the story and exposes a far more sinister man behind the charming and heroic façade MacDonald portrays. The work that McGinnis did became the 1983 best-selling “Fatal Vision” (Signet).

Foley, who was born two years after the murders, was not aware of the story when the studio sent his agent the script for “Final Vision.” Even before the script was passed on to him, Foley started doing research into the case.

“I went back and read Joe’s book and a book called ‘A Wilderness of Error’ written by Errol Morris, and I watched a bunch of interviews,” Foley says. “That was for the benefit of playing my character so I had an idea of what I was getting into.

“I do this kind of research not only once I get a role but for every audition. I have to feel like at an audition, every other guy sitting there, I have done more preparation than them. That I’ve done the work.”

That process has worked well for Foley, as the Kansas native has been performing in TV shows and films since 1995. His credits include “Dawson’s Creek,” “Felicity,” “Scrubs,” “True Blood” and his current series, “Scandal.”

Annable agrees with Foley that the job of an actor — especially when they are playing characters based on real people — is to pull together as much information as possible so each decision made in the portrayal is based on as much truth as can be found. The limitation for Annable was while there’s a lot of material on MacDonald, there’s not so much on the writer. His greatest resource was reading all the books McGinniss had written and using that to get a picture of how the writer worked and thought.

Foley could have reached out to the real MacDonald, as he’s still serving a life sentence for each murder at a prison in Maryland. But McGinniss died in 2014. The two actors share the fact that neither has played roles based on real people and that is different because true stories comes with knowing a lot about where the story begins and ends.

Both stressed that despite all the information available about the case, they could not allow that to taint the full performance. Annable says the important thing to do with a role based on a real person is to play each moment as honesty as possible. For him, that meant McGinniss entered into the agreement to write the book with an open mind. That slowly changed — as did his performance — as more information became available.

Annable, who is a fan of true crime stories, decided the key to playing McGinniss was curiosity.

“I tried to take every scene my character was in and he was just curious. He was trying to get the story. He was looking into this guy’s soul while he was talking to me and trying to figure out who he was,” Annable says. “Journalists see the world in a very different way and I tried to tackle that curiosity in this case.”

All this acting knowledge comes from years of work by Annable. Just like Foley, Annable came to “Final Vision” with a long list of credits, including “666 Park Avenue,” “Red Band Society,” “Heartbeat” and “Brothers & Sisters.” Foley and Annable had not worked together before “Final Vision,” but they did have an acting link. Foley’s wife, Marika Dominczyk, played Annable’s girlfriend on “Brothers & Sisters,” so they’ve known each other socially for years.

As for adhering to the honest of each moment as it unfolds, Foley took on his role with the same approach as Annable.

“When I am playing MacDonald where he is first telling McGinniss that he’s innocent and needs the writer to follow him around, I can’t be thinking at the end of the movie I am going off to prison,” Foley says. “I have to throw all the knowledge away for the moment.

“But, it’s that way in every film. When you do a movie, you get the whole script and you know what you are going to say. But, you have to be as honest in each moment as possible without telegraphing the next moment.”

If the story of the murders and MacDonald’s trial sounds familiar, it was made into a miniseries in 1984 with Gary Cole playing MacDonald and Frank Dent as McGinniss. The difference between the two projects is the miniseries looked at the overall events, while the new movie is told from the point of view of McGinniss that is far more definitive in the question of guilt or innocence.

Immediately following the broadcast of “Final Vision,” Investigation Discovery will air “Jeffrey MacDonald: People Magazine Investigates.”