I know that some characters actually go to DU to learn how to be that character (Disney University). They also learn how to autograph the particular character - and ALL the autographs have to be exactly the same. Basically, it's learning how to forge.
As for me, personally, I have been the Maleficent in 2
MNSSHP on the 31st of October - the "holiest" of nights. When I showed up at the first one, Disney CMs thought I was their Maleficent because of my costume (see avitar). They had me play Maleficent in 99 and in 00. After being swamped by kids and by some overgrown children (i.e. adults) - I was given a chaperone in 00. I was given a "crash"

course in being a character - not to speak (though I DO have an evil laugh and was allowed to use it), be nice and have countless pictures taken with people, but I was told, or rather ORDERED NOT to sign autographs, since I didn't know what and how to sign. I was also not allowed to "de-head" in public. My chaperone took VERY good care of me.
I can tell you that I stood for over 500 pictures with people. The most memorable event was when a little girl of about 7 came over and hugged me to stay as I was getting ready to take a break - the inside of my rubber mask was a bit stifling). My chaperone told her mom I'd be back in about 15 minutes. Neither the Mom nor the girl seemed to understand. The girl pulled on my gown and wanted me to look at her - that's when I noticed her hand movements. She and her Mom were both deaf and using sign lannguage. I signed back to the girl and her Mom and suddenly I became even MORE the center of attention because Maleficent was signing! That warmed the cold cockles of Maleficent's heart that night.
On a side note, I went scuba diving at EPCOT the next day and the little girl was there watching. I went right over to the window and started signing to her and telling her about the fish and everything.
I think when I am in costume (as well as others who play their parts) - I have more fun and get more out of the interaction with people, then those I/we meet.