"Rogue One: A Star Wars Story" film has a companion canon prequel novel, "Catalyst" by James Luceno.
Should "Catalyst" be read before or after watching the film and why?
(the question assumes that the story will be read, one way or another)
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The relationships and storylines have more depth, more flavour, and more tragedy, if you've read the backstories of everyone involved.
Catalyst details Galen Erso's relationship with Orson Krennic and his involvement with the Death Star project. It goes into detail about why Galen was so critical to the project, and why Krennic was so determined to bring him to work on it.
Krennic's whole involvement with the Death Star, and his relationship with Grand Moff Tarkin, are also delved into in much more depth in the book.
Jyn Erso's early life, and more of her mother Lyra's personality and importance, are covered in detail.
It's not essential to understand the story of Rogue One, but it does provide a lot of context and depth to the story the film tells without spoiling anything. You know the endgame that some characters have in mind, and the information in Catalyst can add whole new interpretations to some sequences.
With that said, don't put off watching Rogue One unnecessarily just because you haven't read Catalyst; I don't think the film would suffer if seen without having read the book.