I think first and foremost the main thing to remember when writing Anya is that while sometimes she can talk too much or say the wrong thing at the wrong time, she always has a reason.

For example, in The Body Anya asked questions that, by themselves, were cold and even a little crude, and while there were better ways she could have gone about it, she wasn’t asking to be rude or gross. She simply wanted to understand. Anya had lived for so long never having someone close die that Joyce’s death and everyone’s unwillingness to talk about it left her confused and distraught.

I think the biggest oversight people make when trying to write Anya is that she never lies. That’s not true. Sometimes she’s awkward about it (like when she knew Dawn was the key) and sometimes she’s a pro (she kept her engagement a secret for months).

And she likes to be the one that states things bluntly. She's proud of it and there are many times when it comes in handy because she's the only one who will say how it is (beacuse let's face it, the Scooby Gang is great but they keep more secrets than Nixon).

There really aren’t any rules that I’ve seen to differentiate what lies she’s better at concealing so it’s pretty much at your own discretion. Just remember that while she doesn't like it, she knows it's necessary and for a good reason she'll do it.