3. The Shapeshifting Speech
Ralph Fiennes was obviously having a lot of fun the day(s) they shot Voldemort's final attempt to take Hogwarts in Deathly Hallows: Part 2.
In the same scene as Draco's hug-surprise, Fiennes also dropped in another few improvisations during his monologue in the courtyard in front of the Great Hall.
Here, he has come to profess his victory over the coward Harry Potter, laying claim to the magical school and informing all who remain that he will accept them with open arms (literally, in Draco's case), if they cease their resistance and join his ranks.
Drawing on his talents honed on the stage, where lengthy monologues must be learned by heart and delivered over and over again, Fiennes had no problem going again, take after take, to get each shot in the sequence and produce the best possible scene for one of the film's critical moments.
Everyone else, however, didn't necessarily have the same staying power.
Thus, he regularly delivered an assortment of new lines, directed at different actors, to keep the cast and extras perpetually on their toes, embedded in the moment and acting as if this were the first time they had encountered the Dark Lord in the flesh - and we have one of Voldemort's defining moments to show for it!