Recitatives and Ariosos

This might be a dumb question but I'm trying to figure out how recitatives and ariosos are put together. After scouring the internet for a while all I can get is definitions but nothing that says whether they're written, improvised, or a little of both. When I'm watching an opera it's hard to imagine a composer writing down a conversation somehow in time with the music but it also seems strange to think that these sections are just made up as they go along. For something like Don Giovanni I could see it being just some chords written under the words without an exact rhythm or melody given but I can't imagine anything not being written out perfectly in something like Elektra. I don't know, this has got me stumped and I can't seem to find the info anywhere. Any help is appreciated.

Recitatives and Ariosos

Other Stuff