You want it so when you click on someone an action occurs, so you probably want to start with the mouse action, there are several mouse flags that detect mouse action such as
NPC Code:
mousedown
as Lance mentioned, then you probably want it to detect if a player is there so the final action doesnt just pop up out of nowhere even when there's no player there, so you can use
NPC Code:
testplayer(x,y)
that'll return the index of the player at x,y.
now depending on what you want the final action to be, you can use different commands, you can use say2 as you mentioned.
p.s. soo many run-ons