Rian Johnson has let slip an industry secret about how the presence of an iPhone in a movie could serve as an early indication as to whether a character is going to be a hero or a villain.

Speaking to Vanity Fair, the Knives Out director shared a helpful tidbit for moviegoers, as he revealed that Apple imposes a strict rule that prevents their products from being used by villainous characters in movies.

"I don't know if I should say this or not," he admitted. "Not 'cause it's lascivious or something, but because it's going to screw me on the next mystery movie that I write, but forget it, I'll say it. It's very interesting. Apple, they let you use iPhones in movies but - and this is very pivotal if you're ever watching a mystery movie - bad guys cannot have iPhones on camera."

Johnson then joked that "every single filmmaker who has a bad guy in their movie that's supposed to be a secret" will now want to "murder" him, as he has divulged the details of a prop spoiler that could potentially help audiences to figure out the on-screen motivations of certain characters before they are officially revealed by the movie's plot.

While it is unknown whether any other companies operate a similar mandate over product placement, fans will now be donning their sleuthing hats to identify which character is using an Apple-branded product in Knives Out 2, though they will have to try and not get distracted by the whole new cast, mystery, and location that has been proposed for the sequel.

Adele Ankers is a Freelance Entertainment Journalist. You can reach her on Twitter.