Before and during a criminal trial, the prosecution must hand a variety of information over to the defense. While they need not explain exactly what they plan to argue to the jury, usually the information prosecutors have to disclose (for example, trial witness names and statements) reveals their case theory. Regardless of what they've disclosed, though, they generally can't hide their theory of the defendant’s guilt until the last minute. Consider the following example.
In a 2013 federal appeals case, the defendant was on trial for the murder of his wife. The defense argued during trial that someone other than the defendant was the killer. The prosecution proceeded on the theory that the defendant committed the act. But just before closing argument, after the prosecution and defense were finished presenting evidence, the prosecution came up with a new theory, that the defendant was an aider and abettor: He might have killed the victim, but he also might have aided and abetted the person who did it. The judge gave the jury an instruction on aiding and abetting, thereby allowing it to find the defendant guilty on that theory. The jury convicted the defendant.
The appeals court found that the conviction was improper, that the defendant’s Sixth Amendment and due process rights were violated. Defendants are entitled to know the nature of the accusations they face so that they have a “meaningful opportunity” to defend against them. When, as here, the court allows the prosecution to effectively ambush the defense with a new theory, reversal of a conviction may be necessary. (Smith v. Lopez, 12-55860 (9th Cir. 2013).)
To read more about the prosecution’s discovery obligations, see Discovery: What and When the Prosecution Must Disclose.