For most character meeting, you simply show up and join the end of the line. For Frozen and Princess gatherings, there will be a cast member at the end of the line checking tickets so that only ticketed guests may join the line. I've only been to one Frozen gathering, which was in a dining room (allowing for a decorated, snowy environment). The Princess gathering has usually been in the main lobby, so everyone can see the princesses but only those with tickets can go up to meet them.