Interesting question Beth.
I think that there are several ways of accomplishing this.
The first of course is to simply paint a very simple one color background with possibly a value gradation of one or two values darker or lighter going vertically or horizontally.
If you had a more complex background that the person was intended to be sitting in, then you could go there and paint that background from life. Or, you could set up a background in the studio and paint it.
Or, more likely and much more convenient to the artist and the subject would be to take pictures of the subject against your intended background. Pull the subject in to paint them from life, and then paint the background from the photographs.
The thing is, IMO, it is not important that you paint the entire painting from life, but that you get at least some time with your subject so you can observe them and paint what you see into your picture rather than relying only on what you see in the photographs.
Hope this helps.
Hybrid Mode
