But why rehearse? All it takes is a dab of Mentholatum in the eye. Howard Kurtz @ WaPo:

Some staffers say they have watched rehearsals, on internal monitors, in which Beck has teared up or paused at the same moments as he later did during the show. Asked about this, Balfe responded sharply: “Glenn reacts the same way to issues whether he knows people are watching or not, and is proud to show his emotions, unlike the cowardly, two-faced critics who hide behind anonymity.”