I don't usually get involved, with my luck it will be some train wreck, with amputations, posturing, ejection and a crowning ob in the back, so I take out my handy dandy cell phone and dial the magic number.
However I believe some professional courtesy should be extended to a provider who off duty stops to assist, its it going to kill you to be polite.
The problem is, egotistical, arrogant and ignorance are prevalent in EMS to the point where at times a slap up side the head is warranted.
I always thank a provider or bystander who gets involved. Then again I'm not on a power trip, and I don't get my rocks off insulting good samaritans.