^Same. Some of mine are because RP means getting to fix what I hate about canon (I'm still miffed about Kristen being killed off, the show could have done so much more with her!).
I like to play them because, while you have canon to refer to for tone and behaviour, RP gives you wiggle-room by putting them in unfamiliar situations!
Advice would largely consist of: keep them recognisable. Maintain flaws and tey not to use RP to sanitise out any undesirable traits unless you do it through character development.