I think it varies a lot actually.
J. Tillman is usually a drummer/back up vocals and on his "solo" albums he plays guitar and sings lead and is the main focus. His stuff is acoustic so on a few songs it is just him and an acoustic guitar and on some there are drums, etc. A few of his "solo" albums pre date his work with Fleet Foxes too.
A lot of "solo" stuff is material an artist does that is different or not along the lines of stuff that would fit in with their band(s) sound. Also a lot of the time all the music was written by the artist but of course they can't sing, play lead guitar, bass, drums, keyboard, whatever, whatever, all at the same time live. Quite a few do for studio sessions though.
No artist is really truly "solo" unless they are like a dj or something.
Anyways, you should still check out J.Tillman's "solo" stuff lol.
www.myspace.com/jtillman