There are numerous books on the topic, and every composer, librettist, director, singer and audience member has their opinion as to what role the music should play in an opera. In a play without music, the words ought to support the meaning behind a character's actions. However, in the case of action movies, often times the action is what tells the story and there aren't words, just images to forward the plot. We might hear music to accompany action, build tension, but the action tells the story. Musicals tend to speak until the emotion of the moment just "has" to break out in song - unless it's something like "Mama Mia" where the dialog is the rails we travel on between songs; the songs themselves, written before the concept of the musical, are the focus of the production, while the story is secondary.
Operas are a different beast. The music provides emotion to the words spoken, but it also plays the role of ambience when there is only action. In "Peter Grimes" the court room scene has a fair amount of music to convey what everyone is thinking as Peter enters. It is mood music as we might expect in a film. Because the music is continuous in opera (even silence is part of the music - John Cage) it also needs to flow from one section to the next. It's not to say there can't be jarring moments when the music takes a dramatic shift, but in the end all the music should feel as a single piece, one continuous thread. Often this is one of the prime elements composers fail at when writing an opera. They will write an aria with a lovely melody, but not really consider how that fits with what comes before or after. Linking it into the opera doesn't happen and so, while it may very well be a beautiful piece of music, it breaks the line of the music which breaks the flow of emotion for the audience.
Richard Todd, of The Ottawa Citizen, gave a review of a performance of Ambroise Thomas' Hamlet. Mr Todd states, "The opera's music can't always convey the necessary range of action or emotion without some visual support." While I don't know the opera, I suspect this has more to do with an overabundance of words, rather than the music's failing. In my review of "A Burial at Thebes" I felt the words actually got in the way of the music. Had Dominique Le Gendre removed some of the poetry of Seamus Heaney's libretto, she might have been freer to explore the emotion through music. Unfortunately, what we were left with was an opera where the characters had so much to say and no time to allow the music to process the emotion.
Ian Rankin, who is working on an opera, Gesualdo with Craig Armstrong says "the job of the music to get to the hearts of the people, and the words to get to the brain." During their initial work for Scottish Opera's Five:15 Rankin commented on the need to trim words, again and again. The initial performance (last February) showed an opera that allowed the music space and the words impact.
Margaret Garner, an opera by Composer Richard Danielpour and novelist Toni Morrison, premiered in Detroit in 2005. Mr Danielpour feels as if music is the driving force behind opera, "The most amazing thing about Toni is that she understood from the get-go that in an opera, music has to drive the drama... The composer is the dramatist." While the story was Toni's, Richard obviously feels the drama is his.
I don't agree. The music conveys much of the emotion, but if the words don't have an element of the emotion already in them, the music just comes off sappy and over the top. Some suggest opera is an over the top art form, but I'm not sure it has to be. Music also is the ambience behind scenes with no words, but again the music needs to be married to the action or (as in film) the scene will feel awkward. When working with film, the point of the music is to enhance the film, but (for the most part) be transperant, invisible, there but not noticed. If the music takes focus, the action fails and so does the film. I feel that music needs to play much the same role in a modern opera. The ambient portion of the music needs to be invisible, to the point the audience walks away affected by the scene, but not necessarily humming the music.
The arias should have the opposite effect. They need to have memorable melodies. The brain responds to music different than it does to words. A easy way to learn words is to put them to music. If the aria has a memorable melody, than the words and thereby the sentiment of the aria is also more memorable. But there is also the danger of being too song-like and venturing into music you would find in a musical, a lovely tune, but not one that flows out of the music prior. So, in many respects the music is the glue that binds the words and actions into a complete experience. In order to do that the music needs to play a variety of roles and not just one.