Orson Welles used to talk about “divine accidents” as essential to great filmmaking. He called the director “the man who presides over accidents.” And other artists, like Peter Bogdanovich and Noah Baumbach, have echoed the notion. Along similar lines, improvisation can be similarly crucial to authenticity. Living in the moment, as an actor, is a virtue. And finding reality on a film set is actual magic. Here are some examples — the greatest improvised movie moments.