Our favorite princess interactions still go to
Merida - Last Jan, my (almost 4 at the time) daughter told her that we had a "NO BEARS!" sign on her bedroom door that kept bears away from her room. Merida thought that sounded like an awesome idea, so the two of them sat down and made a no bears sign together that she then hung up on her castle wall.
Ariel - On thanksgiving last year, same daughter was wearing her Hanna Ariel PJ's, which had her signature all over the pants. Ariel asked my daughter if she could get her a pair of pajamas with her picture on the shirt that said Noelle all over the pants
Favorite non-princess interaction
Noelle told Druzilla she was a "big fan" of how mean she was to Cinderella, and asked her for tips on how to be meaner to her little sister. (She laughed until she was almost in tears) Then they had a sourpuss-off to see who could make the meanest looking face for a picture.
Edit: Runner up, this one was all Noelle and not the character. She met Mulan last year, and Mulan asked her to show her best "warrior pose" For some reason Noelle decided to use this opportunity to stick her belly out and run around shouting "PREGNANT PREGNANT PREGNANT" (her preschool teacher was pg at the time, and the kids were obsessed with it)