If an officer stops you out of the blue and wants to ask you questions, you are not legally obligated to tell them anything. This is true whether you are or are not actually under arrest; if you have simply been stopped on the street or even at your home by a police officer then you can answer whatever questions you like and skip any you feel like skipping. You are not under arrest and as such you can walk away at any time. If the officer feels that speaking with you is of particular importance or that you are acting guilty of something, they may stop you again right away but the same rules apply: unless you are being arrested there is no obligation on your part to answer any question at all.
If you are under arrest, however, there are really no differences except that if you do choose to answer questions the answers will be noted for future reference and perhaps use in court. When you are arrested, you will be told exactly what your rights are in terms of answering questions so that no confusion occurs. Basically, you do not have to speak to police (as with a non-arrest situation) and if you choose to have a lawyer present when dealing with questions you are fully entitled to do so. If you do not have a lawyer or can’t afford one, the State will provide one for you.
A lawyer may not seem a necessary provision when speaking to the police, but the face is they will know what questions might cause you to incriminate either yourself or a friend so if you talk to them beforehand and have them present during the discussion you can avoid letting something slip that might hurt your case. This is only in extreme cases, however, since for the most part a police officer will only want to speak with you briefly about something that you may have a slight knowledge of, say a work colleague’s whereabouts on a certain night or something of that nature. Again, giving out information like this can seem like the wrong thing to do, especially if it concerns someone else who is unaware of the current questioning, so if you do not want to answer such questions you are within your rights always to refuse to do so.
The key thing to remember if you don’t want to answer police questions but you also don’t want to end up in unnecessary trouble, is that if you seem evasive they probably will come back to try to talk to you again. Don’t run away, don’t be very rude and hopefully your slight uncooperativeness will just seem like a way to keep your privacy and maintain the trust of others above the strangers that are the police. Whatever information you wish to divulge is up to you – there are no questions you must answer and if you want to answer none that is completely your call.