My question is tangentially related to this one, but I'd like to take it a step further. Let's assume I'm an outside consultant and I'm working with a product or system that I have very little knowledge about. How much information do you need about a site's users, goals, domain, and features, in order to reliably evaluate it?
I ask because I've sometimes had people ask me to look at a friend's website or something on the fly and, though I'm doing my due diligence, I often give them the thumbs up only to notice a deeper issue a month later. As a UX professional, I find it's already hard to get people to take my advice seriously (as in, actually do what I recommend), and getting egg on my face like that just makes it harder.
A great answer will give me ammunition to give to a client if they ask me to do a quick HE. That is, I should be able to say "I'd love to do a heuristic evaluation for you, and in order to do that properly I should first do x, y, and z..."