During the scene where Jake Gyllenhaal screams at himself in the mirror, Gyllenhaal got so into this improvised scene that the mirror broke, cutting his hand. He was driven to the hospital by the director after a nineteen hour day of working and got forty-six stitches in an four hour long operation, returning to the set six hours after being discharged. This was the reason why Gyllenhaal had his hand behind his back in the scene where he tells the scrapyard owner his motto.
Director Dan Gilroy on the impetus for the film: "I think to some degree it's certainly an indictment of local television news, but I'd like to cast a wider net in the sense that all of us really watch these images. I would hope that maybe a viewer would take it further and maybe go, 'Why do I watch these images and how many of these images do I want to put into my own spirit?'"
The filmmakers made a point of not having Lou Bloom undergo a "character arc" because they felt he would have become a certain type of person and stayed that way as an adult. That was also why the initial scene has Lou assaulting and robbing a security guard; it was important that the audience not feel Lou became a worse person because of his work as a nightcrawler, but instead recognized he was malevolent from the start.
The Raishbrook Brothers (the actual "stringers" Gyllenhaal and Ahmed trained with in preparation for the film) make a small cameo when Louis arrives to the structure fire too late near the beginning of the film. They are the two last ones to leave the scene. They would later go on to make and feature in the Netflix series Shot in the Dark (2017).
Dan Gilroy considered a backstory that would help explain how Lou Bloom became the amoral and damaged "survivor" he was in the film. However, he ultimately cut the entire idea from his script because he didn't want to either spend a lot of time on Lou's origin or make him very sympathetic.
After the scene in which Lou shows the newsroom his Horror House footage, the broadcast director cuts to break on a Bird's Eye food commercial featuring wolves. The movie follows this theme of predatory animals in its depiction of Lou, and the commercial ends with the slogan "Dinner is Complete," as the Horror House story is Lou's big break into the world of stringer journalism.
The film's climax was filmed on Laurel Canyon Blvd. in the San Fernando Valley, only several blocks away from the location of the infamous shootout in 1997 between two heavily armed bank robbers and L.A. police, in which both robbers were killed.
Despite receiving nominations for Best Actor at the Golden Globes, BAFTAs, SAG, Critics Choice awards, Independent Spirit Awards and even the Saturn Awards, Jake Gyllenhaal failed to receive an Academy Award nomination. Critics, audiences, major film groups and publications considered this a major snub at the time.
When Lou and Rick are in the car listening to the police scanner for the first time, dispatch requests a code 2 (lights and sirens) at "Sixth and rampart". Rick suggests they respond to that and Lou laughs back "We want victims... Not the kind that live on Sixth and Rampart". This is a reference to the 1976 black comedy film Car Wash staring Richard Pryor, filmed at an actual car wash located on the corner of Rampart and 6th street in Los Angeles.
The creators consulted with an LA-based "Stringer" organization called RMG News, owned and operated by Howard, Austin and Marc Raishbrook, who also feature in Netflix's original series 'Shot in the Dark' (2017). Two of the brothers appear very briefly as extras in the film after Lou arrives late to a fire.
During a Q&A, director Dan Gilroy stated that he views Lou's insane drive as an "infection" that spreads to each character Lou interacts with throughout the film. At the closing of the film, where Lou's company vans are parting ways down different streets is, as Gilroy describes, "the infection spreading through the veins of the city."