They shot the film for close to six months, only at night.
He went on multiple ride-alongs with real paramedics to research the character and at one point had to pick up a victim in Hell's Kitchen who'd been "shot through the ass." He was worried that the kid was going to choke on the gum he was chewing, so he even took the gum out of the victim's mouth while the real paramedics were prepping him for the hospital.