I think that usually the composer wrote the music first and then a writer would write the libretto. Is this so? I also remember reading that it could happen the other way around, a composer writing a opera to an existing libretto. Was this common?
I'm particularly interested in the creative process behind Mozart and Verdi works. Did they set the tone and devise a plot while composing the music, or was it mostly defined by the writer in the libretto? Did they ever contribute with lyrics or dialogue? Who had the final word in putting the music and libretto together?
Thank you :)
You have it backwards! The libretto always came first. I actually can't think of anyone who wrote a whole opera and then tried to get someone to write poetry that somehow fit the whole 3-act thing, that'd be just nuts. An overture could be written in advance, maybe recycle a few arias, but that still leaves the recitative to be set to music, the recitative being the bulk of the words in an opera. So, as a whole, you have to start with the poetry. Libretti are very beautiful pieces of Italian poetry in their own right, they follow complex rhyme structures and styles, and were written by well-educated poets of the time, they really do reserve more respect than they get these days. Opera, from its birth until maybe the mid-19th century, was understood expressly as poetry first, and the music was to help you enjoy the poetry.
Opera composers often had little input in the libretto, but Mozart and Verdi did collaborate with their librettists. Unfortunately I'm very rough on Verdi, I can maybe hash you a fair amount on Mozart's working relationship with his most famous librettist, Lorenzo Da Ponte. I can talk about baroque working conditions until you sneak out the back though, that's my real specialty.
The plot of a libretto was usually based on an existing story, like a historical story, a mythical story, or sometimes a play, Marriage of Figaro (likely the most popularMozart/Da Ponte opera today) was based on a wildly popular play. Think the new Frozen musical coming to Broadway. It's actually a bit debated what their working relationship was like - did Mozart boss Da Ponte around? Did Da Ponte boss him around more? Hard to tell from the output today! The opera shows elements of both of their talents, it is easily demonstrable that they did work closely together when making this opera, more closely than many. This popular article on Da Ponte might be interesting to you.
Generally the composer had the final say in a libretto, they could ask for changes to make it fit their cast needs better, or just if they were struggling to set something they could ask for something more suitable. If they were setting a very old opera (like a Metastasio) where they were not working with the original writer, they would usually be working with a librettist who would adapt the opera to their needs. But then, the composer would be subserviant to the impresario, the director, most of all. The composer (and likely the impresario) would however, especially in the 18th century, be largely subserviant to the needs and tastes of the singers.
If you're interested in some reading, this book is very good, and covers premiere operas of both Mozart and Verdi. It's a really beautiful book too, like a coffee-table book almost.