Characters appear on Main Street USA randomly throughout the day.
In the past we have seen lots of Disney pals appear, usually for a very short time.
Winnie the Pooh is permanently up by Casey's Corner, and there is a permanent area outside the Boarding House in Town Square, which is usually for Duffy, but other characters do appear there too.
During various holiday seasons, Main Street is the place to find special characters too. For example Elves at Christmas time and Leprechauns on St. Patrick's Day.