Paramedic here: Very inaccurate. An arrow is considered a low velocity weapon causing injury only to the immediate area involved (as opposed to a high velocity weapon such as a rifle bullet that will cause internal injuries some distance from the wound due to internal shock waves and cavitation). If the arrow was to strike a major blood vessel or vital organ, it could start copious bleeding that could be fatal after several minutes (and cause unconsciousness within about 60 seconds). The only type of arrow shot that would cause immediate or almost immediate unconsciousness and death would be a direct shot into the heart (and even that would need to be in one of the two ventricles for immediate death) or directly into a vital part of the brain or upper spinal cord.

This is not saying that an arrow can not be lethal. An arrow hit in any major organ or blood vessel will kill the victim, but not immediately. I would assume (and hope that an armament expert can chime in) that the arrow was used more to immobilize or injure ones opponent and take them out of the current battle.

To tack onto this question, movies often show bowhunters being able to quickly shoot and re-arrow (think Legolas). How accurate is that?