What has Rosencrantz done to make Hamlet say this? It’s not as if he’s not realized Rosencrantz is there; He just greeted them with “My excellent good friends.” Plural. So it’s not as if he’s suddenly surprised to see Rosencrantz. He’s just asked Guildenstern a question but he doesn’t wait to hear Guildenstern reply, his attention suddenly turns to Rosencrantz and then to them both.
Has Rosencrantz suddenly gestured in a “What am I, chopped liver?” sort of way? Has he pulled Guildenstern out of the way? Has he made a funny noise? Thumbed his nose and stuck out his tongue.
I do want him to have done something. It’s just more interesting than a generic “Ah.”