I Wish People Wouldn't Make Life So Difficult

RP problems. I've dealt with people who are slightly difficult regarding roleplaying, with the ones i've dealt with - it's the whole character leaving in the middle of the storyline thing. There has to be a better response than just "not interested" "not gonna be involved in this" or anything like that. Also when the RPer changes their characters backstory and species without warning me first. Ugh..like for example how can you go from being a cat-person to a vampire to demon in a matter of seconds after making a deal with the devil?