Many of Longlegs' encrypted messages contain misspellings and poor grammar after being decoded. This is an homage to the Zodiac Killer, who was also known for his coded messages containing typos, which often made it more difficult to decode his message.
To promote the film, Neon created a phone number (4586664355) which, if dialed, played a prerecorded, extremely disturbing message from Nicolas Cage as the killer.
During an interview, Osgood Perkins recalled a story from production where he learned Nicolas Cage has a particular skill that he says no other actor possesses: the ability to recognize how high or low he is able to speak without messing up the audio. According to Perkins: "The sound guy came over to me one day...(he) comes up to me a couple of days into Nic being on set and he's like 'Oz, I've never seen anything like it. When Nic is mic'd, I'm watching the dials, when Nic goes big, he goes *right* to the line. Anything more, a decibel or two over that, and it would be hard to use. Then he goes down, he goes soft and his whispering and he's barely talking, he goes right to the line. Anything past that line, you wouldn't be able to use it. He knows where the lines are. It's the craziest thing I've ever seen in my life'."
Despite the praise and acclaim of his portrayal as Longlegs, Nicolas Cage has stated this will be his only portrayal of a serial killer for film. In an interview with IndieWire, Cage said "I know that the phone's going to be ringing off the hook to play serial killers after Longlegs. And that's not really what I like to do. I don't like violence. I don't want to play people who are hurting people."
The request of Osgood Perkins of not using Nicolas Cage on the posters or in the trailer despite him playing the titular character led Neon to come up with a creative plan to promote and sell the movie without depending on Cage.