Voldemort
-call him Mr. Riddle (yes, Mr. Riddle; please, Mr. Riddle; thank you, Mr. Riddle...)
-tell him Dumbledore is more powerful than him, plus he's got hair and a nose.
-send him a singing Valentine...tell him it's from Wormtail.
-ask him where he wants to be buried after he's dead
Hagrid
-tell him Norbert and Aragog were dangerous
-keep asking him questions until he lets out a secret
-wonder out loud your suspicions about his umbrella
Hermione
-imply that divination was a hard subject for her
-offer to give her your house-elf
Lockhart
-tell him he's got something between his teeth
-ask him for an autograph, then say "never mind" and quickly walk away
-pretend to almost recognize him