Once in the hubworld, there are a few areas where the characters might be. I think their locations can be random.
First, do a lap of the semicircular area where you start and approach each character you see. Pressing
initiates conversation with them for this achievement. Also, whilst you have them press
to also read their bio for another achievement.
From the starting area, go up each tube and do the same in those areas (they all have more than one area inside, doors are pretty blatent).
Also, grab any Pellets and Pies you see for the 50 lives achievement (more info there)