One of my favorite oxymorons: "greatest weakness." When in an interview, your job is to present all your strengths, why you're a great fit for the job, and how you could make an immediate impact to the company's success. But often during the discussion, you're asked about your faults, your skills lacking, your downside.
The first advice you often hear on this subject is to use a strength, but to an extreme. The most (tired) popular answer: I'm a perfectionist. Another: I work too hard. Many interviewers disregard this type of answer and ask for another one. And by the way, sometimes being a perfectionist is a true weakness. Sometimes, close to perfect is good enough when in a hurry.
Others will advise you to use humor. Making fun of yourself shows humility. For example, you could say, "My spouse tells me that I am horrible at reading maps." I suppose as long as you are not driving around town for your job, they won't care. However, since this isn't an occupational weakness, the interviewer might ask for another one. You'll have to abandon the humor aspect and give a more serious answer.
One other approach interviewers take is to simply say they don't have any weaknesses. This, of course, is nearly impossible. So in essence, you are practically lying in your interview -- always a bad thing.
The interviewer wants to know that you recognize you have faults and where you might miss the target as an ideal candidate. They know well-prepped interviewees will not provide any reason to be dropped as a viable candidate. So in essence, it's more of test to see how well you present your case.
So what should you say? Pick a real weakness from your occupational tool set that you know has no bearing on the job you are interviewing for. The interviewer will acknowledge that you were honest with them, and will likely consider your "greatest weakness" to be meaningless to him. You should also mention that you are studying or taking a class to improve in this area. For example, a computer programmer might say that she wishes to improve her public speaking skills and is attending Toastmasters for training. By including your plan for overcoming this "weakness," you have actually turned your answer into a pseudo-strength: You recognize your faults and set a plan for self-improvement. A very good quality.
A similar approach would be to discuss a weakness that you have recently overcome. Be sure to be able to talk about how you identified it, how you overcame it, and what you learned from the experience.
Yes, there is a bit of a "song and dance" when it comes to interviewing. It can seem tedious. But until the tune changes, you should know the right steps so you don't trip.