What does "You would play upon me; you would seem to know my stops;" mean?
This play on words that Hamlet utters to Rosenkrantz and Guildenstern (IIIii, ll. 364-5) shows that he is not fooled by their false friendship. The whole speech is a musical metaphor;"pluck" refers to lutes, a recorder (flute) has "stops" or "frets" --holes that vary the pitch of the notes. When Hamlet says "Call me what instrument you will, though you fret me, yet you cannot play upon me" he is continuing the metaphor, using the players' presence and recorder to make his point. ("Fret" is a play on words also.) He is saying that, if they cannot even play"this little organ," a recorder, how can they expect to manipulate as complex a person as Hamlet? The metaphor was introduced earlier (IIIii, ll. 70-71) to Horatio, a true friend: "That they are not a pipe for Fortune's finger/ To sound what stop she please."