I’d acknowledge them, but I’d let them know I was en route to an appointment. Depending on if I considered it a good idea I might talk with them later. I wouldn’t let them stop me from going to the appointment, but I wouldn’t play head games, either.

It would be totally dependant on the situation. Very hard to say without context.

Whats wrong with honesty? I'd say hi, tell them I can't chat right now because I've got an interview to go to, and ask if we can catch up later...