So I remember a few years back, reading an interview with Eddie Van Halen. He was talking about this one time that a fan had said to him that she saw four people on the album covers, but only heard three. He then said how he was trying to explain what the bass is and what it sounds like, but he didn't feel like he was getting the point across.
If it were me, I think that I would just play them the intro to "
Another One Bites The Dust". It explains pretty good what the bass does...provides rhythm & harmony and you get to hear it in its fairly typical range.
So how about you? If you had to explain what "that guy" does in a band, what song do you think would do it? Or if not a song, how would you explain it?