The burden of proof is initially on the plaintiff in a personal injury lawsuit. In order to prevail, he or she must prove each of the four elements of negligence (duty, breach of duty, causation, and damages) by a preponderance of the evidence. This is usually done through a combination of expert witnesses and lay testimony.
For instance, in many car accident cases, the basic facts of the crash may be explained by lay witnesses (“I saw the blue car run the red light and hit the side of the white van”). Evidence regarding certain damages, such as injuries and the reasonableness and necessity of medical expenses, requires testimony from an expert witness such as a physician.
In the middle of these extremes are cases in which expert witness testimony would prove helpful but is not strictly required in order for a case to go forward. Sometimes, the parties disagree as to whether such testimony is an actual requirement under the circumstances, and the court must make a decision.