In the most recent thread about Star Wars, there was a discussion about some plot weaknesses, one poster said “You should read the EU [the books set in the SW universe]” and another responded “Maybe the EU is fun, but if you NEED to read it to make sense of the movies, the movies suck.” I think that can lead to an interesting discussion - how standalone should a work be to be ‘good’ in general? Here’s my take:
I think that a movie, TV show, comic series, book series, or the like should be standalone in it’s primary format - all of the major points should get made ‘onscreen’ or ‘onpage’. If there are spinoff books to a movie, it’s fine if they explore the backstory, worldbuilding, other characters, and the like, but it shouldn’t be the case that there are major parts of the main work that make no sense without buying spinoff books, comic books, and playing the video game. It’s perfectly valid to judge a work’s quality on it’s own, and if it has a fault then the fault can’t be removed with ‘well, if you play the third video game and take the ‘evil’ path you’ll see…’ or ‘if you read this later essay by the author, he changes his mind’ (like Heinlein with Starship Troopers).
In general, secondary works should be standalone aside from the main series, unless they’re clearly just ‘reference material’ (like the Dune Encyclopedia or Star Trek technical manual). They might reference the main work without explanation, but their own stories should be solid enough to make sense without cross referencing multiple other works. Exactly what is a primary work, secondary work, or multiple independent primary works is up for some debate, but generally a single series should stand on it’s own - if the Lord of the Rings trilogy didn’t work without the Simarillion or The Hobbit, then it would be fair to criticize it for that.
If you’re making an adaptation of a movie, the adaptation should bring enough from the main work that someone can make sense of the adaptation without having read the main work. For example, a Dune movie doesn’t need to do a deep dive into the Harkonnen and Atreides interfamily conflicts and historical roots, but should establish that they are have been enemies for a long time. It’s fair for an adaptation to leave in weaknesses of the original, like the way the LOTR movies don’t address the old ‘why not just fly into Mordor on the eagles?’ question any more than the main books do.
What are other people’s thoughts, and any noteworthy examples of this working or failing?