It depends on the situation. You are allowed not to answer questions that would cause you to incriminate yourself, IE, 'did you kill that man', and your refusal to answer cannot be taken as evidence that you have or have not committed the crime. On the other hand, if you do not answer a question, where you are not the accused, it can be considered obstruction of justice. This leads to an interesting dilemma, since potentially, you can be asked a question when you're not accused of a crime, where you're compelled to answer, but that testimony can be used later as evidence in a trial against you. That situation is called Entrapment, and is technically illegal, though it's difficult to argue either during questioning, or at a later trial where you are being accused of a crime, that entrapment took place. If you lied, your crime is purgery, if you told the truth, you essentially admitted to wrong doing. Additionally however, it's difficult to prove a person knows something, and the last interesting factor is that the law isn't entirely rigid. If an officer pulls you over and asks why you were swerving across the lanes of the highway (assuming you're not intoxicated, true story incidently) you can take the 5th, or you can explain your situation, and the same situation will have different results. In my case, the situation was that I was tired after a concert, driving an unfamiliar road, and the lane markers were scratched out due to construction, so I didn't know where I was supposed to be driving until they reappeared, and I was straddling two lanes. If I had taken the 5th, I almost certainly would have been hauled down to the station for drug screening. Since I explained that I was tired, and only a few minutes from home, (and there was no scent of alcohol on me) the officer let me go, following me until I was off the highway.

Not necessarily, but a persons degree of possible guilt, would race to the moon. if a person is not guilty, why not respond? hmmmmm?.

It depends on the situation and the circumstances. you do not have to answer any incriminating questions, without an attorney being present. this is what the miranda warning is about. At a later date, if it was discovered that you had information, concerning a major crime, you could be arrested for obstruction of justice. this information would be presented to a grand jury and their decision would stand.