Your Miranda Rights are named after the U.S. Supreme Court case, Miranda v Arizona, 384 US 436 (1966).
The police must advise suspects of their Miranda Rights -- their right to remain silent, their right to an attorney, and the right to an appointed attorney if they are unable to afford counsel -- prior to conducting a custodial interrogation. If a suspect is not in police custody (i.e., "under arrest"), the police do not have to warn him of his rights. For example, if the police contact you by telephone, you have no plausible argument that you are in custody during your conversation with the officer over the phone and thus cannot claim a violation of your Miranda rights.
The police are very aware of when they have to read suspects their Miranda Rights. The police will frequently question a suspect, specifically telling the suspect, "You are not under arrest, and are free to go. However, we would like you to answer some questions." After the suspect voluntarily answers questions, and sometimes if he refuses, he is arrested. The questioning, being voluntary and non-custodial, is usually admissible. After arrest, the police may have no interest in further questioning, and thus may not ever read the suspect his Miranda Rights.
Sometimes a suspect will make voluntary statements after he is arrested. The police do not have to warn suspects not to make voluntary statements, as long as they do not deliberately try to elicit those statements through statements or conduct. Sometimes, suspects will express their surprise at being caught by the police, with statements to the effect of, "You got me." At other times, suspects will try to justify their actions to the police after they are arrested, with statements such as, "I don't know why I did it," or, "The drugs weren't mine -- I was carrying them for a friend." Those statements, if made spontaneously by a suspect, will almost always be admissible in court. Additionally, if a statement leads to the discovery of other evidence, even if the statement itself was taken in violation of the Miranda ruling the police may be able to use that evidence.
When a person chooses to remain silent after receiving his Miranda warnings, that silence cannot be used against him in court. However, if a person has not received his Miranda warnings, and remains silent, it is possible for that "pre-Miranda" silence to be used against him. For example, if a person is arrested for murder, or is told that he is a suspect, a typical innocent person will express disbelief and may even try to present an alibi. It would be unusual for a person to simply remain silent, after being informed that he is being wrongfully charged with murder -- even people who know their right to remain silent will often express surprise. A prosecutor may subsequently argue that the "pre-Miranda" silence resulted from the fact that the defendant was not surprised that the police "figured it out."
If you are under investigation for a criminal offense, you can prevent "pre-Miranda" silence from becoming an issue by stating, "My attorney told me never to talk to the police without talking to him first. Do I have to answer your questions?" Once informed that you have the right to remain silent, no negative inference can be drawn from your exercise of that right. There is nothing wrong with making your attorney responsible for your choice to remain silent -- it looks a lot more suspicious if you simply refuse to answer questions than if you present the explanation that your attorney gave you standing advice not to answer questions.
The police tend to draw a negative inference from the fact that suspects refuse to answer questions, or where suspects hire attorneys ("lawyer up") before they are charged with crimes. However, there are many cases where the only evidence against a defendant is his confession, or where an innocent person finds that the police have misinterpreted his statements. In one notable case, a police officer was a criminal suspect -- he made a taped statement, expressing his innocence. Subsequently, he was shocked to hear his tape recorded "confession" used against him in court. As it turned out, his statement was recorded on a used tape, which contained a confession from a different case. Part of the old recording, immediately after the end of the police officer's statement, was presented as the defendant's "confession." If that can happen to a police officer, obviously it can happen to you.
If I Choose To Remain Silent, Or Request An Attorney, But Later Decided To Answer Questions, Can They Use My Statement?
If the police do try to question you after your arrest, they are supposed to cease interrogation if you exercise your right to remain silent or request an attorney. It should be noted that the request for an attorney is "more powerful" than a request to remain silent. Courts tend to view police claims that a suspect changed his mind about having an attorney with much more suspicion than claims that the suspect changed his mind about remaining silent.
The police use numerous techniques to get suspects to change their minds about remaining silent. One very simple technique is to use silence against the suspect -- the officer explains, "You don't have to make a statement, but I still have to write up this report, describing what everybody says that you did." The officer, in front of the suspect, then starts to type out his report, saying nothing to the suspect. It is common for the suspect to break the silence, and to choose to make a statement.