Do singers ever sit down? Do classical singers in a chorus, opera or recital ever sing sitting down? Why not?

In an actual OPERA, there may be scenes where a singer is seated when they sing (and even lying down). That's because they are acting out a story, and it makes sense for the story that the character is seated. In this video of clips from the opera "Peter Grimes", you see the singers seated several times during the drama.

https://www.youtube.com/watch?v=axtuFTb2Cv0

And "Carmen"--the mezzo-soprano sits hunched over a washing bucket at one point in this famous aria. In fact, she sits down quite often in this PARTICULAR production.

https://www.youtube.com/watch?v=K2snTkaD64U

Here in the opera, "Nabucco", you see most chorus members seated while they sing. There are a few people lying down, though in this clip, I can't tell if all the people lying down are singing or not (some are). A few may be what is known as a "supernumerary", an "extra" that is usually unpaid and does not sing or speak lines (yes, even in opera there are occasionally spoken lines), and is there to basically fill out a crowd scene or provide atmosphere.

https://www.youtube.com/watch?v=4NF6LweEA_A

Once in awhile, if the chorus is not seen by the "audience", then they may be seated, as in a choir or organ loft during a church service. However, it IS easier to sing while standing. It's not impossible to sing seated, but it is easier to stand, and in concert situations, I think it looks more formal.

Then for practical reasons, if a gentleman in a very formal evening dress (white tie and tails) or a lady in an elegant evening gown, were to SIT, then the garments are likely to wrinkle, drag on the floor, get in the way, etc. I think it seems more professional in a way to perform on one's feet. After all, we don't see rock stars SITTING on the stage during their performances--not even the ones that don't really dance.

There was a pop singer in the fifties called Perry Como whose "thing" was sitting on stools. He would be dressed casually in a cardigan sweater (the adjective "casual" was often applied to Como). A television show parodied Como's relaxed way of singing ( The real Perry Como didn't always sit, and sang up-tempo songs along with the slower songs he "crooned").

https://www.youtube.com/watch?v=zj5A7W-0zPY

I sing in a church choir. I sit between hymns but always stand to sing. The voice projects better when standing.

It compresses the diaghram, making it hard to sing,