Here's a question: is a movie a musical if it is only ABOUT music, but not one where characters spontaneously break out into musical numbers?
So, is That Thing You Do a musical? Most would say yes because it's filled with musical numbers and is about the music industry, but all the numbers are performed as stage shows or "jams" - they are a resultant aspect of the plot that is supposed to be happening as depicted within the "reality" of the movie, but they are not necessarily a narrative for the plot.
(Whereas Gene Kelly "Singing in the Rain" could be construed as part of the narrative - is it really happening or is the music just an interpretation of his inner thoughts & feelings in that moment? And if it is really happening, then where is the orchestra out on the street on a rainy evening?)
Are movies like Amadeus or Mr. Holland's Opus "musicals" since they contain music and are about music? Or are musicals strictly those which contain "musical numbers" that are not presented as actually taking place as part of a performance within the reality of the story, but are a narration of character's thoughts or situations set to music that occurs spontaneously?
If music made films musicals, then any films with scores would be musicals.
A film can be about music and feature music without being a musical.
The looser we are with the definition of the category, the less meaningful the category becomes. If we are talking about a musical, we want to set it off from other films as a "musical" and not just "another movie," so we do need an exclusionary definition, one which will have some borderline cases that some will feel are on the "wrong" side. If you don't have some tough cases, then you almost certainly have an overly expansive (i.e., useless) definition. The tough cases, therefore, are not only the test of a category definition, but also proof, in themselves, that a
prima facie requirement of such a category has even been met (i.e., to have some discriminating power, sorting this from that).
In short, it is BOTH feature and bug to have tough cases.
In my book,
it is not a musical if the actors do not sing dialogue and/or soliloquies as a "reality" (i.e., getting the depiction of the characters "really" talking to each other through song and/or expressing their inner thoughts spontaneously through song). Since this is not a feature of our lived experience, this means that musicals have, by a strong expectation (if not logical necessity), a significant unreality to them (i.e., talking and expressing through song, continually). A film that does not meet this minimal definition is not a musical.