Like Hoku said, you can't guarantee seeing specific characters but I can tell who you MIGHT see at certain locations based on our recent trip.
DLR:
- Princess Fantasy Faire has 3 princesses. We saw Snow White, Cinderella and Sleeping Beauty.
- Pixie Hollow has Tinkerbell and, when we went, Iridessa and Vidia. The latter two apparently switch off so you're only really guaranteed to see Tinkerbell. DD5 wanted to go try again to see her other favorite fairies but I didn't want to waste the time in line again.
- One morning around 11 am near the Castle's side paths, I saw Ariel, Sleeping Beauty and a third princess (blocked by line of kids) on their "Royal Walk". You can ask CM about these from what I hear.
- At Tangled, we only got Rapunzel but I have heard the guy is sometimes there, too. Sorry, I forgot his name.
- One afternoon, as we entered the park, I saw Minnie on Main Street in the plaza right by Town Hall.
- In Toontown, we saw Minnie in front of her house and Mickey in the back of his house (long line to get to him and he was wearing his sorcerer outfit). We also saw Pluto on the street.
DCA:
- I saw Goofy in a hawaiian shirt near Goofy's Sky School.
- I saw green army men at one point somewhere near Little Mermaid (I think they were wandering as opposed to a set station).
- While we were on Mickey's Fun Wheel, I saw Woody and Jesse posing for photos under some sort of covered awning area.
- At Ariel's Grotto, we saw Ariel (guaranteed as she greets every diner as you walk in) and 4 other princesses: Belle, Aurora, Cinderella, and Snow White
- There is a spot to meet Duffy but he wasn't there when we walked by.
I'm not counting character sightings where you can't walk up to them and talk or pose for a photo with them (e.g., Soundsational Parade, Aladdin musical).
P.S. We were there for the start of Halloween so they also had meet-n-greets with villains (e.g., Cruella de Vil, the cats from Pinocchio, Captain Hook, etc.) at a staged area near IASW and on the path near Haunted Mansion.
HTH!