Dan Wells, author of the mystery-horror novel I Am Not A Serial Killer that was recently adapted into an independent film, was allowed to spend over a week on the film set, a rare honor in the film business.
At the 2016 DragonCon screening of the movie (you can see my the spoiler-free review of the movie here), Wells said most of the time writers of the source material aren't included much. He cited the case of one writer who was only allowed on-set one day and not allowed to talk to anybody. However, the director of IANASK was a friend of his. So Wells got to be on-set ten days and talk to the cast members about their characters. Speaking as someone who'd love to see his books adapted into movies (Battle for the Wastelands could easily be a cross between Gettysburg and Lord of the Rings), this would be a high honor indeed.
Furthermore, the film's low budget--$1.2 million--gave them a lot more creative freedom than a big-budget film with a lot of studio money people involved did. Executive Meddling is a common problem in the film business, especially if there are concerns the filmmakers' vision might alienate audiences. With costs so low, it's easier to make a profit and thus more risks can be taken.
During a Q&A session after the movie, I asked Wells about why the movie looked so old-fashioned. He said it was shot on FujiFilm that's no longer made. The director of photography had two movies' worth of this film stored in a refrigerator in his garage. They wanted an "old, gritty-looking" feel to the movie and this they got. I read elsewhere the film was shot on 16mm film. I'm not familiar with how movies shot on different media look (although I did notice when I last saw Dog Soldiers that it looked in part like it was shot in the 1970s and apparently older Dr. Who episodes were shot at least in part on video), so I guess that's why movies shot in particular time periods look the way they do.
I also asked why the killer didn't attack John when John came across him killing another character and Wells said it was clearer in the book that the killer was too weak to do both.
Wells also said he wrote the book he wanted to write and the book served as a test for finding "his people." Wells paraphrased how they'd react below:
"I thought this was a crime novel. Holy cow, this is a monster. This is so cool!"
The audience also learned that half of the I Am Not a Serial Killer extras also worked on the television series Fargo. Given that both stories take place in the cold Upper Midwest, this is a pretty good choice.
Before the film, Wells' Writing Excuses cohort Brandon Sanderson (yes, the one who finished the Wheel of Time series when Robert Jordon died) interviewed Wells. They spoke about their long friendship and the easygoing competition they have with each other. That got the audience a tale of the origin story for IANASK--Wells worked for a scrapbooking company and read serial-killer biographies to ward off the excess of kitsch. When Sanderson gave him a ride home, he regaled him with the MacDonald Triad, which informed the first line of the first draft.
"There are three traits common to serial killers. I have all of them."
Wells also said that he wanted his adaptation to be different from the book. After all, he's written the book already and knows what's going to happen. For example, John's love interest (as much as he can love) Brooke in the novel is blonde, but the actress is brunette. Although he was skeptical at first, after seeing her performance it was hard for Wells to see Brooke as blonde afterward. And although he and the director had some "friction" over how the movie had a different ending than the book, Wells did say it looked good on film and overall, it was fun to see others' interpretation of his ideas.
Finally, we also learned the first version of the script had a lot of voice-over, reflecting how the novel was told in first-person POV. However, this ended up cut out--people will assume the protagonist is safe. Furthermore, Wells said that actor Max Records, who plays Cleaver, conveys all the V.O. stuff through his facial expressions anyway.