It’s not uncommon for a script to face any number of obstacles on its way to becoming a film. Between scheduling conflicts, casting issues and plain old creative differences, at times it seems like a miracle that a movie is ever finished.

In bringing Deliver Us From Evil to the screen, director and co-screenwriter Scott Derrickson dealt with his share of setbacks, including a title change – the horror thriller was originally called Beware the Night -- a shifting release date that led to a tighter production schedule, and the loss of leading man Mark Wahlberg, which opened the door for Eric Bana to step into the role.

Inspired by the real-life accounts of NYPD veteran Ralph Sarchie, Deliver Us From Evil stars Bana as a skeptical Bronx cop who’s pulled into a case that a renegade priest (Edgar Ramirez) convinces him involves possession by evil spirits.

Derrickson, who wrote and directed The Exorcism of Emily Rose and Sinister and will soon turn his attention to Marvel’s Doctor Strange, embraced the subject matter.

“There's a world outside of our understanding,” he said during a recent meeting with journalists, where he was joined by Brana and Sarchie. “I think the big forces in the world, science and, frankly, religion, are a lot of times kind of broadcasting this idea that we have everything figured out, and whatever we don't have figured out we have a theory for that's probably right. I don't think that way. I don't think it's true, and I think that believing in mystery, embracing the massive mystery of the world is something that adds to the excitement of living and deepens one's faith and deepens one's experience in the world. So those are the kinds of stories that I'm interested in telling.

“I love to tell stories that remind me, in the process of making them, that I think the world is a magic place. I walk through a magic world and I don't want anyone to talk me out of it, unless it's true and I don't think it is true. I think the world is magical.”

However, it was Sarchie’s doubts about the supernatural that drew Bana to the character.

“I liked the fact that the character was initially very skeptical, because I thought it was better for the audience particularly, and potentially a lot scarier, to follow someone who doesn’t believe in it at all, and slowly he’s dragged into that world and forced to have to deal with it,” Bana said. “I thought that was better, more interesting because ultimately I think that makes the film scarier in the end.”

Although Sarchie spent a lot of time on the film set, he tried his best not to interfere with Bana’s performance. Still, the two managed to forge a bond.

“[I spent] every day with him, with the exception of maybe four or five scenes,” Sarchie recalled. “The scene where he was in the chapel. I figured that might be a difficult scene for him, so I stayed away. Not only that, I didn't want to be there. I wanted to save something for when I sit down in the movie theater.”

“He was on set as our police adviser,” Bana added. “That was his actual job on the movie. He was around in pre-production, so we got to hang around together and spend time with him. I chose not to use Ralph for [the role] because it just didn’t feel like the right thing to do for me. I just wanted him to be himself and go about his business. I was always going to selfishly steal something from him anyway just by osmosis, I guess. There are a couple of things I took away from him, but in order for him to do that I needed him to just be himself and not just sit down and tell me stories for hours and hours. So I just kind of let him be and we got to know each other pretty well.”

While it’s debatable whether Bana’s performance was enhanced by Sarchie’s presence, the two didn’t spend much of their time discussing acting.

“We both ride motorcycles, and 90 percent of the time we’d just be talking about bikes, bike gangs and stuff like that,” Bana said. “That was kind of our way of getting to know each other and we felt very comfortable around each other. We relied on him for all the police procedural stuff and the technical stuff on set, and he was very good at that.”

Audiences will likely leave the theater talking about the lengthy exorcism scene involving Bana, Ramírez and the suspect played by Sean Harris. It’s powerful and well-executed – and it may have been the most physically and mentally draining aspect of the shooting schedule.

“That was our final week of production,” Bana said, “so as tricky as the shoot was with locations and weather and everything, we knew that we had this gift waiting for us at the very end, which was this week long exorcism in the studio, which was the only thing we shot in the studio, actually. We knew it was going to be hell and pretty full on and Scott gave us a lot of warning about that. It was insane. I think, I’m going to say four days, for that one scene.”

“It's a really complex scene to pull off, part of it because we were in such tight quarters,” Derrickson added. “The idea was to trap these three enormous personalities into this one space with this conflict, but it was a technically difficult scene too. There was a lot of makeup changes, which means every time something happens, you've got to stop and reset the makeup. I think the impact of it in the end came down to those actors and them being prepared.”

He praised the talent and stamina of the actors who faced the brunt of that final week.

“Edgar, having done his homework about what he would be doing exactly as an exorcist, and, most importantly, Sean Harris just delivered on that scene,” Derrickson continued. “He really went to a crazy, dark place in the second half, to the point where it was upsetting for people on set. He was retching between takes and he was in some kind of weird trance state. People were nervous for his mental health shooting that scene, I'm not kidding. It shows because when he begins to speak, and he starts to speak the Spanish lines from the point where there's the stigmata. There's not a human presence behind his eyes or in the way he communicates. It is alien. It is so transcendently dark and that's just what great actors can do.”

Naturally there must be a few changes made to the original story to give audiences what they expect from a supernatural horror film. Unfortunately for the real Ralph Sarchie, that included making some modifications to his past.

“The one scene that I'm really interested -- I mean I've seen it already, but I want to see it in the movie -- is the part where the rosary is there,” Sarchie said. “I called Scott one day and I said, ‘Scott, do me a favor and put the rosary in this movie.’ He says ‘Done, and I'm glad you called me,’ and he did it. He's been very accommodating with the things that I have asked for, and a little shocking with some of the things that I discovered, like, me being a murderer.”

“That was from the script! Because he read the script, so he knows everything that's in there,” Derrickson said. “It was really funny, because when we changed the title from Beware the Night, which was the title of his book, to Deliver Us From Evil, which was a marketing decision, I remember he called me up. He was like, ‘Scott, come on! You made me a murderer! Can't you at least let me keep my title?’”

Deliver Us From Evil opens today nationwide.