Just a few thoughts, in my experience:

1- Call 911 (if possible) and give the most accurate and detailed description of where the emergency is at. For example, N/B direction of ---, east of ---. etc.

2- Pay attention where you park because the emergency vehicles will block you in for as long as they're there. Don't park near a fire hydrant if there's a fire.

3- If there are injuries, don't move victims unless you absolutely have to- you could make their injuries worse. I recently responded to an accident where the unbelted driver wound up on the passenger's belly. I told the passenger not to move and to leave the driver there. The passenger was hysterical and wanted me to do something- he called me everything in the book. Turned out the driver sustained a broken back, punctured lung and broken ribs. I can only imagine if I removed the driver... I don't have very deep pockets.

4- My pet peeve... if there are no or very minimal injuries and the cars can move, MOVE THEM OUT OF THE WAY! There is no sense in leaving the cars there to prove fault of the accident. The damage will more than likely prove that anyway.

5- I'll think of more later, I've got to refill my glass...