I have dressed up as Disney characters and have been involved with playful activity with some of the ones that I dressed up as. I wore a Prince Charming costume one year and the WDW Cinderella was with the WDW Prince Charming. She saw me (I was with my wife and daughter), pushed off of the WDW PC and rushed over to me and said "Where have you been all night?!" That was neat. Another year, I wore a Donald Duck costume and ran into the WDW Donald. He mimicked/mirrored all of my moves for about 5 minutes. That was a "thrill" for me 'cause I am a BIG Donald fan!
In the past, I have dressed up as Mr Incredible, Captain Hook, and Winnie the Pooh. I have never encountered a problem with the cast members and have always been somewhat encouraged (in my opinion) by the CMs as they will call out the characters name.