Answer: She provides useful evidence as an expert witness
Throughout the film, Lisa plays a significant role in the case Vinny is dealing with by helping him. She also keeps Vinny informed about facts that he isn't aware of. Although she doesn't play a very active role in the trial initially, her testimony proves to be instrumental towards the end of the film in proving Bill and Stan innocent. She is herself a mechanic, and belongs to a family of mechanics, so she is able to effectively testify in the court as an expert witness. A photograph that she has in her possession proves that Bill's car couldn't have been used by the culprits to escape, since the car Bill was driving doesn't have the features that would have made the tire marks possible, such as an independent rear suspension, posi-traction and flat tire marks. Vinny questions George Wilbur, who endorses what Lisa tells the court, and the sheriff also testifies that the real culprits were arrested and found driving a green coloured Pontiac Tempest and were found to have a gun which was of the same type that had been used to shoot the store clerk.