Immediately following the discussion of plan,Hamlet appears. Wouldn't this be a good time to try out the plan ? Do they?

Claudius entreats Rosencrantz and Guildenstern to try to discover the reason for his distemper (so he says). Rosencrantz and Guildenstern soon enter again, surprising their friend Hamlet. The three friends banter philosophically for a good while before Hamlet asks the two why they have come to Elsinore. Rosencrantz and Guildenstern try to dodge this question, declaring that they have come for no other reason than to visit him. Hamlet, though, won’t let them off the hook, and makes them admit that the king and queen sent for them. When they admit it, Hamlet also tells them why they were sent for – because he has been deeply melancholy, and has foregone his accustomed behavior. He sinks deeply into a speech detailing this misery.