We prove personal injury claims at trial by presenting evidence. That much is obvious. Although sometimes third parties have important evidence, often the most vital evidence is in the hands of the Plaintiff (the injured person) or the Defendant (the negligent party).
How Evidence is Lost
In the movies, parties to a trial secretly destroy or lose vital evidence to a case that could work against them. In real life, lost or damaged evidence is often the result of more innocent events. For example, a video recorder that records over itself automatically, erasing vital evidence, or a Plaintiff injured by a defective product who innocently throws the product out.