YMMV, but the part where Guildenstern and his voice echoes and everyone just looking around in confusion when they hear the echo, Rosencrantz and Guildenstern especially when they look around, as if to say, "Did I/he just do that?" One example being this piece of dialogue:
Guildenstern: You know what to do.
Guildenstern: Are you stupid?
Guildenstern: Are you deaf?
Rosencrantz: Did you speak?
Guildenstern:(realizing what he's doing) Not now.
Guildenstern:NOT NOW!(echo and he and Rosencrantz both look around, as do all the players)
A brilliant little moment poking fun at the increasingly philosophical story: "Death is a state of non-being! You can't not be on a boat!" (Beat) "I've frequently not been on boats."