How can I get people to answer truthfully?

I am working on a project that requires me to ask questions of total strangers on the street. one of the biggest challenges I face is that I can't seem to find a way to get honest answers. Well, I think that some of the answers are honest, but I can't always tell. Can you suggest a way for me to get honest answers from total strangers? Thanks!

When you ask a question, try and have a few follow-up questions to shoot out at them right away. For example: If they answer "this way" to your question have a few questions to give them "quickly" that will cover the angle they are going down. Be ready to ask the follow-up questions quickly, because it is harder to lie quickly.
If they answer another way, then have a few questions ready for that angle, and so on.....

In other words don't just think of a question and ask it....Think of a question and then think of all the possible ways to answer it, and then "think" of a few questions for each way it may be answered. Try it and let me know. This will make for a good interview and I will guess that if they are lying your going to catch them....Don't forget the follow-up question fo that one!! lol. Let me know.
Familycoach

Show them a gun....preferably close to their head. People will respond to that.

Tell them, in order for the project to be accurate - you must answer honestly (it is anonoymus) - will you ? - you won't be individually identified I promise.

I've done some reading about this exact problem. On one hand people tend to be more honest with strangers than friends and family. On the other hand people still get embarassed I read that in certain countries there is such a stigma to homosexuality that no AIDS patients would admit to having homosexual contact. The way around this was instead of asking them if they had homosexual contact ask them about specific practices that are common among homosexuals but rare among heterosexuals. The subjects were not as embarassed answering these questions truthfully.