I’ve got a favorite book (an older book) that I think would make a great movie. I’d like to write a screenplay based on the book, but I really have no idea what the legal process is or should be. I’m not sure how to find out if someone else owns the rights to adapt the book. Even if no one else does own the rights, I’m in no position to acquire them yet. Is it permissible to complete the screenplay and then secure the rights?

Thanks in advance for any guidance you can offer. I’m a writer by trade, but I’ve never written a screenplay (well, not for commercial purposes, anyway - I did write one as my senior thesis many years ago) and I’m just not sure how to attack it.