Any good actor will tell you that it learning the lines is crucial to any performance: you have to have the lines down cold to then seem natural and at ease. You see this most obviously when candidates are caught unprepared, or off-guard by a glib comment—the fumbling, searching for words and process time. With rhetoric like “friend,” statistics and surveys, and intense scrutiny of past media engagement, both Biden and Ryan intensely rehearsed for their night in the national spotlight. And even their stage is meticulously set against any surprises: favorable lighting ensures neither candidate’s face is cast in shadow, tie decisions are checked against both suit and set design, and the candidates are kept in air conditioning set at 53 degrees to ensure neither sweats profusely (a trick learned after the first televised debate in 1960 between Kennedy and Nixon—in which many watchers thought that the overly-warm Nixon was uncomfortable due to the being bested by Kennedy, while radio listeners tended to think that Nixon came off better than Kennedy. Kennedy eventually won the race, but the debate became a crucial talking point).
Samuel Yates, It Doesn't Matter Who Won the Debate