Here's the secret to answering "What is your biggest weakness?" I've used it several times with great success. Are you ready for it?

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Tell the interviewer what your biggest weakness is. Be honest with yourself, and then be honest with them. I don't mean that you should blurt out, "I stay up all night watching Gossip Girl reruns," (unless that affects your productivity at work in a non-trivial way), but do an honest assessment of yourself, decide what your biggest work-related weakness is, and tell them.

If you end up working with them, they're going to find out what it is anyway, and if they're aware that you're aware of it, they can help you overcome it. If you just grin and say something like "I work too hard" or "I'm too much of a perfectionist," you've only succeeded in wasting yourthe interviewer's (and your) time.