Lawyers are bound by ethical rules that prohibit them from offering evidence to a court that the lawyer knows is false. Knowing a client plans to lie requires the lawyer to take action to stop that from happening.
Telling your lawyer you plan to lie under oath places them in a terrible spot. It also puts your defense in a predicament. Plus, what you're considering is a serious crime (perjury). Anyway you look at it, there's no good outcome.
Criminal defense attorneys sometimes have competing duties. They must 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 Professional Responsibility Rules, doing so subjects the lawyer to discipline—and quite possibly, disbarment. (Rule 3.3, ABA Model Rules of Professional Conduct.)
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. It not only puts the lawyer in an ethical bind, but it can also hurt and upend your defense strategy.
Your lawyer can't assist you or a witness lie to the court. The lawyer will advise you on the consequences of lying, from weakening your own defense to getting caught and facing a much tougher sentence or additional perjury charges. If you surprise your attorney by lying on the stand, your lawyer will be left unprepared when the prosecution undoubtedly attacks you, your story, and your credibility. Your lawyer also can't refer to any false testimony in closing arguments—another limitation on your defense.
Lawyers work with guilty clients all the time. Their job isn't to prove a criminal defendant's innocence. Rather, it's to protect their constitutional rights and make the prosecution prove its case against a defendant beyond a reasonable doubt. Knowing all the facts of the case, even the bad ones, can help your lawyer develop a defense strategy to cast doubt on the prosecution's story and prepare you for tough questions.
Attorneys whose clients stubbornly insist on offering perjured testimony face a difficult dilemma, pulled between their duties to the court and their client.
To avoid violating the attorney-client privilege, your lawyer may ask to withdraw from the case without saying why and ask that another lawyer be appointed. When such a request is made, a judge may suspect that the reason is the defendant's intention to commit perjury, but many other reasons could 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 their 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 come 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, defendants and witnesses who perjure themselves face the possibility of a criminal charge of perjury, which is a serious felony.
Our justice system is an adversarial system designed to seek out the truth. Lawyers zealously argue, defend, and advocate for their clients, but they also have a responsibility to uphold justice as officers of the court. Knowingly presenting falsehoods to the court (even if it's through their client) undermines the integrity of the justice system. It's also important for lawyers to have options when a client insists on lying; otherwise, a client could strong-arm the lawyer into presenting lies and falsehoods to the court.