I've spent a lot of hours job shadowing an ER doc and where I'm located at least the days can definitely vary. Some days are more hectic with ambulances bringing in major injuries where the ER doc is busy resuscitating patients or patching them up to send them off to a surgeon. Most days seem to be more relaxed with the doc sitting on his computer for a few hours waiting for beds to open up while tests are being ran. A lot of what they see are what patients would normally go to a family doc for like a fever, strep throat, or a hernia but their insurance will pay if they go to an ER instead of seeing a PCP. Obviously all this is going to vary significantly depending on the area you live in.
EDIT: This is solely from a job shadowing point of view