If I met one of the Scoobies, Andrew, or Spike, I would definitely start up a conversation and try to become friends so I, too, could be a Scooby!
If I met Angel, I would quickly put on all black clothes, look dark and broody, and go up to him. I would pretend to be a messenger of some kind and warn him that if he did not stay away from Buffy, very bad things would happen. I would make sure he understood, and tell him he could do more good in Cleveland than Sunnydale. And then I'd tell Buffy he left and set things up so she could find solace in the arms of Spike! Mwahahahahaha!
If I met a villain, I'd run and hide, because they'd probably kill anyone who talked to them!
If I met Kennedy, I'd probably have a reaction similar to most people's reaction to meeting Willow.
And if I met April, I'd give her a hug, because she needs one badly.