I saw Willie Nelson performing on some late night show.
There appeared to be someone in the band playing an instrument with eight strings. I paused the playback and could only see eight strings.
I know nothing about music but this struck me as curious. What instrument might I have thought had eight strings.
Possibly an Eight-string guitar?
A MANDOLIN ?
There are classical mandolins with round backs, and newer flat-back instruments, both with 8 strings.
They are small, tuned in 5ths the same as a violin, but with pairs of strings. They are usually played with a flat pick.
Without seeing the picture I can’t tell, but I’ve been playing in bluegrass bands for 35 years with mandolin players.
There are also custom guitars with 7, 8, or 9 strings.
I hope this helps.
David
How did he play it?
Standing? Sitting?
Strumming? Picking? Plucking? Tapping?
Are the strings evenly spaced or paired?
Better description would help.
Possibly it’s a mandolin. A mandolin looks like a slightly misshapen half-sized guitar; it’s got four pairs of strings (from a distance it might look like four thick strings), and has a high, almost tinkly sound (the guy might play the same note a very quickly a bunch of times in a row, because each note doesn’t last very long). It’s very common in bluegrass, and often in country music.
The band was on The Late Late Show with Craig Ferguson last week and they did have a mandolin player, so I’m sure that’s what you saw. Here’s the whole episode. I don’t think I can link to the musical segment, but it’s the last part of the show.
The musical performance is at about 36:30, actually. The last segment is just the very end of the show.
Yes, one of Willie’s backup musicians (second from the left) is playing a mandolin in this segment. He takes the first solo, in fact.
Bouzoukis also have eight strings.
As a pointless addendum to this: one would think a mandolin is pretty commonly recognized instrument, but I play mandolin and lap steel in a band and I am quite often being asked between sets the names of those weird instruments I am playing.
Didn’t you also want to ask why Willie has a big honkin’ hole in his guitar next to the regular hole? It’s for his stash!
Wait till you put telescoping legs on your lap steel, then it’ll really blow people’s minds. 
“But… it has… legs. It’s not in… your lap!”
The Americana/Appalachia sound has caught on here and I know a handful of dobro/resnoator/lap steel/banjo/mandolin/fiddle/whathaveyou players, but yeah, I wouldn’t believe “most” people would know what a lot of these things are. Even though well played lap steel is a thing of true beauty.