My daughter just returned from 2.5 months being a Coaster "roadie", "valet", etc. (Intn'l College Program). Actually there are 13 different positions on Coaster and she did them all.
She just told me that being "Chris" is a task that the computer assigns which is different than filling a position. It doesn't come up very often. This is what she told me:
"I went in the studio with the black les paul, i pretended to play the drums, then when he asked chris to bring it in I got up, grabbed the guitar and left."
So if you ever see a CM do this, you're lucky as it doesn't happen very often!