Hopefully I'll cover SOME ground that others haven't...
Each park has a number of permanent character greeting areas...some indoors, some outdoors...where you can find characters 80-90% of the day.
For example, at the Magic Kingdom:
- Mickey Mouse is at the Judge's Tent (backyard of his house) all day in Toontown.
- There will always be a group of characters available in the Toontown Hall of Fame. Usually they have 2 or 3 characters grouped together like Pooh, Tigger and Eyeore. Lines can be a drag, but you know what to expect at the end.

- Ariel appears most of the day at Ariel's Grotto in Fantasyland.
Now, aside from these permanently established areas, characters are often found greeting guests around "their" attractions. Look for:
- Buzz Lightyear outside the Space Ranger Spin.
- Goofy at Goofy's Barnstormer.
- Minnie Mouse at her house.
- Pluto in the backyard of Mickey's house (near his doghouse).
- Aladdin and Jasmine near the flying carpets.
- Tweedle Dee and Tweedle Dum near the Teacups.
- Jessie and Bullseye in Frontierland.
Lastly, at select times during the day, you will often find a group of characters appearing out in front of the castle or in the courtyard as you walk onto Main Street past the Railroad station.
MGM and Animal Kingdom have their own versions of the above. Epcot does have characters, but they are a little harder to find. There is a "Character Caravan" that runs at fixed times each day. Basically it's a double-decker bus carrying as many as a dozen characters who stop to sign for a period of time. You will find random characters in the World Showcase (Belle and Beast in France, Aladdin and Jasmine in Morocco), but only at certain times of the day.
Fear not. The characters will be out there waiting for you. Actually, you may be the one doing the waiting...
