Holding that an expert witness is prohibited from rendering a legal opinion because it would usurp the District Court's pivotal role in explaining the law to the jury
Holding that the district court erred in admitting testimony of an expert who "stated that he ‘rejected’ the possibility that [law-enforcement witnesses] had lied, and explained various reasons why police officers have no incentive to give false statements in excessive force cases"
Holding that “a defendant who seeks the admission of expert testimony must make an on-the-record detailed proffer to the court, including an explanation of precisely how the expert's testimony is relevant to the [issues in dispute]”