I can think of three really great character interactions off the top of my head - two from a long time ago and one from last summer.
1) Back in 1996, I went to WDW with my best friend as a HS graduation trip. We made it our goal to get autographs from all of the cutest guy characters (at the time this was John Smith from Pocahontas, Prince Charming, Snow Prince, and Phoebus from Hunchback). Well, the Hunchback character meet and greets were outside at Hollywood Studios, and we kept getting in the line and right as we got to the front, the characters would switch out and it wasn't Phoebus anymore, so we'd get back in the back of the line. After twice through the line, just as we were getting close to the front, and Phoebus had just come out, so we knew we were going to get to see him, it started to rain and the characters had to go inside. We were crushed, so we asked the character handler if when the rain stopped it would be Phoebus who would come back out or if it would be a different character. He said he'd check and went inside. Well, a few minutes later, out comes Phoebus, in the rain, and walks right up to us in line and says, in a rather arrogant, Phoebus sort of way, "I hear you ladies wanted to meet me," and winks at us. We about died of embarrassment, but he signed our autograph books and took pictures with us and then went right back inside (because it was still raining).
2) Same trip in 1996, also at Hollywood Studios, my friend and I were eating at the
Soundstage Restaurant. The tables had these yellow paper tablecloths and crayons out so you could draw on the tablecloths. What I remember about it was that it was counter service, but there were characters there too. Well, Genie from Aladdin came over to our table and took some crayons and drew a funny picture of Jafar and wrote "Jafar smells bad" next to it and then drew squiggly lines coming off of the Jafar drawing to indicate how bad he smelled. Then Jafar came over and took the crayons and crossed out the picture of him and drew a Genie picture and wrote mean things about Genie. It was really funny, and they were so in character.
3) Last summer, we took my niece and nephew to the fairies meet and greet. I made them fairy costumes, not specific fairies, although my nephew's costume was definitely modeled off of Terence. We got through the lines and the fairies were so great with them. My nephew was 1 and a half and my niece was almost one, and the fairies just made the biggest deal over them. Tink gave my nephew the biggest kiss on the cheek. And then as we were leaving, a manager saw the babies and just fussed over how cute they were, asked if he could take their picture, and then told us to wait there. He came back and said if we wanted, we could come back later that evening when Terence would be there because Terence would be so excited to see another sparrowman come through because they'd never been visited by an out-of-town sparrowman. So we went back that evening, and the fairies were excited to see the babies again, and Terence made a big fuss over my nephew and complimented him on his acorn hat and was jealous that he had acorns on his belt too.