The most basic principle underlying the lawyer-client relationship is that lawyer-client communications are privileged, or confidential. This means that lawyers cannot reveal clients' oral or written statements (nor lawyers' own statements to clients) to anyone, including prosecutors, employers, friends, or family members, without their clients' consent. It doesn't matter whether defendants confess their guilt or insist on their innocence: Attorney-client communications are confidential. Both court-appointed lawyers and private defense attorneys are equally bound to maintain client confidences.
Example: Heidi Hemp is charged with possession of illegal drugs. At the request of Heidi's mother, attorney Joe Lawless talks with Heidi in jail and offers to represent her. Heidi decides not to hire Lawless, and instead retains Bill Mucho as her lawyer after she bails out. At trial, the prosecutor calls Lawless as a witness and asks him to reveal what Heidi told him in their jail conversation. Lawless cannot testify. Lawless spoke to Heidi in his capacity as an attorney, so their conversation is confidential even though Heidi decided to hire a different attorney.
Example: Same case. Heidi tells her lawyer that the drugs belonged to her, and that she bought them for the first time during a period of great stress in her life, just after she lost her job. Heidi authorizes her lawyer to reveal this information to the D.A., hoping to achieve a favorable plea bargain. However, the D.A. refuses to reduce the charges, and the case goes to trial. Cross-examining Heidi, the D.A. asks, "Isn't it true that you admitted to your lawyer that the drugs were yours?" This is probably not a proper question. Heidi authorized her lawyer to reveal her confidential statement to the D.A. But a statement made for the purpose of plea bargaining is also generally confidential, so the D.A. cannot refer to it at trial.
Example: Same case. Soon after her arrest, Heidi speaks to her mother in jail. Heidi's case goes to trial, and the prosecutor calls Heidi's mother as a witness and asks her to reveal what Heidi told her. Heidi's mother would likely have to answer questions under oath about what Heidi said to her. Most states have not created privileges for conversations between parents and children.
Suppose you discuss your case with your attorney in a restaurant, loud enough for other diners to overhear the conversation. Can they testify to what you said? Yes. Lawyer-client communications are confidential only if they are made in a context where it would be reasonable to expect that they would remain confidential. (Katz v. U.S., U.S. Sup. Ct. 1967.) A defendant who talks to a lawyer in such a loud voice that others overhear what is said has no reasonable expectation of privacy and thus waives (gives up) the privilege. Similarly, people who talk about their cases on cellphones in public places risk losing confidentiality.
Jailhouse conversations between defendants and their attorneys are considered confidential, as long as the discussion takes place in a private area of the jail and the attorney and defendant do not speak so loudly that jailers or other inmates can overhear what is said.
But what about phone conversations, either in person (speaking on phones, separated by a glass partition) or using a pay phone? Defendants must be very careful not to allow jailers or even other prisoners to overhear what they say on the telephone. These people sometimes eavesdrop, in person or on the telephone, and then claim that they were able to overhear incriminating information because the defendant spoke in a loud voice. (Inmates often try to curry favor with prosecutors through such tactics.) If a judge believes them, the privilege is lost and a jailer or other prisoner can testify to a defendant's remarks.
Sometimes, jailers warn a prisoner that phone calls are or may be monitored. That warning alone may mean that phone conversations between prisoners and their lawyers may not be privileged. If a jailer monitors a phone call and overhears a prisoner make a damaging admission to the prisoner's lawyer, the jailer can probably testify to the defendant's statement in court. (For more, see Can a jail record my telephone conversation with my lawyer?)
For perfectly understandable reasons, defendants sometimes want their parents, spouses, or friends to be present when they consult with their lawyers. Does that mean that the conversation won't be considered confidential?
Quite possibly. Defendants who bring strangers (people who are not part of the attorney-client relationship) into a meeting risk losing the right to claim that the meeting was confidential. This means that the D.A. might be able to ask the stranger or even the defendant about what was said during the meeting. However, the lawyer can maintain the privilege by convincing a judge that it was necessary to include the stranger in the conversation. For example, if the third party can shed light on the case or otherwise help the lawyer develop a strategy, that person's presence would not destroy the confidentiality of the conversation.
Blabbermouth defendants waive (give up) the confidentiality of lawyer-client communications when they disclose those statements to someone else (other than a spouse, because a separate privilege exists for spousal communications; most states also recognize a priest-penitent privilege). Defendants have no reasonable expectation of privacy in conversations they reveal to others.
Example: Benny Wilson is charged with possession of stolen merchandise. The day after discussing the case with his lawyer, Benny discusses it with a neighbor. As long as Benny does not say something to his neighbor like, "Here's what I told my lawyer yesterday…," the attorney-client communications remain confidential. Of course, Benny's conversation with the neighbor is not confidential, and the prosecutor can properly ask the neighbor to testify about what Benny told him.
If you want to know what is and isn't confidential given the law in your jurisdiction, make sure to consult a criminal defense lawyer. An experienced lawyer can advise you of the relevant law and guide you through the criminal justice system.