...is always wrong.
Well...how can you guard against answering questions that no one is asking?
First, you can have deep relationships. When we think of apologetics, do we often think of deep relationships? Most of us don't. When I think of apologetics, I think of bow-tied professors behind podiums or contact evangelists working a crowd. But to be honest, the bulk of our apologetic work will take place in deep relationships - family members, lovers, friends, neighbors. And a foundation of deep relationship can give you excellent insight into the angle, the slant, the motive, the essence of the questions you're receiving.
For example, when Amy asks me why I think there's evil in the world if God is good, I know that she isn't just curious and she isn't trying to dodge some more uncomfortable issue. Her question's a question about trust. "There's so much evil and suffering...How can I trust God in the midst of it?" If a random person asks me the same question, I may not have a sense exactly what they're asking. Make sense?
Second, you can almost always ask a question to check. Few people expect you to be able to read their minds (and the ones who do should be paying you for the service) and most people will wait to flesh out their question until they're invited. Don't try to read minds (but if you do and you have success, please warn me) and don't be afraid to let people know you can't read minds. They might even take your interest in their question as a sign of respect.
How about an example of a check question? You can get a little more creative than "Why do you ask?" or "Could you explain what you're wondering about a little more?" If someone asks you if, as a Christian, you believe in the Theory of Evolution, you can ask if they're talking about a smooth Darwinian process or some sort of punctuated equilibrium theory. That's a check question. They may be really interested in talking about a specific scientific theory and how it intersects with faith or they may just be wondering generally if your faith prohibits you from thinking scientifically. The question creates a conversational wedge, allowing your friend to direct the conversation to where their interest lies.
Thirdly, you can watch and listen. What's that familiar saying about eyes, ears, and mouths? 2.2.1. Right. We should be watching and listening more than we should be speaking and smelling (unless you count nostrils separately). This is what distinguishes an answer from a lecture.
Someone may ask you a question to which you have a really snappy answer (like my answer to the problem of evil...to be revealed on Sunday). You start or even finish your answer and get a blank stare or a "What I meant was...". This is almost always a sign that you haven't really understood the question. Your temptation, however, is to believe that your friend hasn't really understood your answer and you must try harder. Don't try harder. Ask a check question.
If we're receiving questions in the context of relationship and are asking questions of the questions we're receiving and are paying attention to the people we're sharing with - if we do all that, we'll have taken huge, lunging strides toward answering the questions people are actually asking, rather than the questions we expect to hear.
Always be prepared to give an answer to anyone who asks you for the reason for the hope that you have, but do this with gentleness and respect