When it comes to non-human characters I like them to be . . . well, not human. This means they probably have a set of attitudes, thought processes and values which are completely alien to us. Not everything, some stuff’s going to be the same, but enough for some of their actions and responses to surprise every now and again.
The Doctor from the BBC’s series Doctor Who is a classic example. Alien. Looks human. Big big heart(s). Saves the world multiple times. But every so often is a bit . . . weird. The current actor playing him, Matt Smith, has got that whole a-bit-too-weird-to-really-pass-for-human thing down perfectly. Tom Baker was also brilliant (although I suspect he might actually be an alien marooned down here on earth . . .)
That little spice of alienness is really important to me. I hate reading urban fiction with vampire or werewolf protagonists where the characters display a completely human array of emotions and reactions. If the author hadn’t told me the character had fangs, super-strength, was centuries old, whatever, I doubt I’d realise they weren’t human at all. Feels like a waste (Although I can’t argue with the success of that genre. Maybe my wrong is everybody else’s right! :) ).
I think this is why I might be so reluctant to write anything from one of my succubus’s point of view. It’s hard to avoid humanising a character and takes away a lot of their mystique.