A film is usually completed many months before release; photography generally wraps more than a year in advance of release. The script can float about for years before release. Scripts aren't secret, with rare exceptions (eg a big franchise film like the new
Star Wars) - they're shred around freely, because any script worth its salt is registered with the Writer's Guild for copyright establishment purposes, so there's no big advantage to keeping it secret. Only film buffs enjoy reading film scripts, the general public does not.
Details about who's attached to a project as a star/director, or senior technician are easily available through trade newspapers and industry newsletters. The 'packaging' of a film ('Seth Rogen to star in The Interview') is the first stage of the marketing, taking place as long as 2-3 years before release, sometimes longer. Basically the way you get a film made is this: Write the script; find some famous actors who like it; get them to sign (largely) non-binding 'letters of intent' to act in it if it manages to attract financing; call up investors (including studios) and say 'do you want to put up the money for this? It's a great script, and I've got letters of intent from Seth and James!'; proceed to the stage of writing more binding contracts once you have money in the bank.
Long story short, there has been plenty of time for NK to become aware of the film and get offended about it. By some accounts the idea was in development since the last decade but got shelved when Kim Jong-Il died.