I told my lawyer I'm planning on telling a lie on the stand. What will happen?
Criminal defense attorneys have a duty to zealously represent their clients and guard their confidences. However, they also have a duty to the court not to present evidence that they know is false, fraudulent, or perjured, whether it's coming from the defendant or a witness whom the lawyer knows intends to lie. A lawyer who knowingly uses or presents perjured testimony risks serious consequences. Under the profession's code of ethics (the Canons of Professional Ethics of the American Bar Association), doing so subjects the lawyer to discipline—and quite possibly, disbarment.
Knowing that presenting false testimony violates their duty to the court, few lawyers will allow it. They will do their best to convince their clients not to testify falsely.
Attorneys whose clients stubbornly insist on offering perjured testimony face a difficult dilemma, pulled between their duties to the court and their client. The correct response is to ask the lawyer to be relieved of the case, so that another lawyer can be appointed—without saying why. When such a request is made, a judge may suspect that the reason is the defendant's intention to perjure himself, but there are many other reasons that would support a defense lawyer's request to withdraw from a case.
From a practical point of view, appointing a new defense attorney may not be any solution at all. If the client reveals his intentions to the second lawyer, the same problem will present itself. Even if the client remains silent, the second lawyer, like the judge, may figure out what's going on. For this reason, some judges may deny the substitution request. In that event, the first lawyer might ask to withdraw and ask that the client proceed on his own. In one case, the Supreme Court approved of an attorney's statement to his client that if he gave perjured testimony, the lawyer would question him, effectively cross-examining his own client and exposing the lies. (Nix v. Whiteside, 475 U.S. 157 (1986).)
Defendants who understand the consequences of telling their lawyers of their plan to testify falsely (or offer witnesses who will lie), draw one obvious conclusion: Don't reveal your plan. But hiding one's intention to testify falsely has grave possible consequences: When your testimony is based on a lie, it may be very hard, if not impossible, for your lawyer to defend you against attacks that will come in the form of cross-examination by the prosecutor. And remember—many times, the truth "will out," even in the most carefully crafted stories. When defendants are exposed as liars on the stand, it never goes well, with the jury or with the judge at sentencing time. Finally, witnesses who perjure themselves face the possibility of a criminal charge of perjury, which is a serious felony.
Read more on the crime-fraud exception to the attorney-client privilege.