I've worked for both a paid and volly dept. Both are pretty small depts and in a rural area. I can't speak for other shifts or when other respond, but when I (captain) am on the truck it's like this:
Driver - AO / Engineer
Front Pass. - Highest rank (comms/siren to and from, usually safety and incident command)
Rear Left and Center - Interior Attack (usually a FF and LT or 2 FFs)
Rear Right - Plug man and ground equipment (fan, ladder, etc.)
Wow I could get all into this, but I'll leave it basic. Like many have said, it depends on the dept S.O.P's, size, scenario, and availble resources.
For those who are screaming the "2 in, 2 out" rule, relax... Our primary engine has a fully electronic pump panel with "set it and forget it". I have my AO set the pump and then, with the plug man, act has R.I.T. If need be I'll work the pump until a seond truck can get on scene.