We have all seen a television show or movie where a lawyer stands up and presents a “surprise” witness. The crowd in the courtroom gasps, the opposing attorney stands up and strenuously objects, but the judge allows the witness to take the stand and the case is won on the strength of this single witness.
Great drama, but nowhere near accurate. You really cannot have any surprise witnesses in a real Trust or Will trial. The same is true of documents, they typically are fully known by everyone before trial. And that is by design, we generally do not want surprises come time of trial. Both our discovery rules and rules of evidence generally preclude “surprise” witnesses and documents from coming in during trial.
And that brings us to the Exhibit and Witness lists for your trial preparation. Prior to trial, you are required to make a list of all the witnesses and documents you intend to use at trial. You then must exchange that list with the opposing party BEFORE trial so everyone knows what to expect. You will also provide the exhibit and witness list to the Court prior to trial based on how the court prefers to receive that information (it is best to ask the court clerk that question). By the way, always be polite to court personnel because they can be really helpful when you have questions on how to prepare for trial.
Even before you create your exhibit and witness lists for trial, you have to disclose your documents and witnesses as part of the discovery process (assuming the opposing party asks you to do so). And anything you do not disclose to the opposing side during discovery cannot be used come time of trial. So hiding the identity of a key witness is a bad idea. Not only is it illegal, it also will prevent you from using that witness at trial. The same applies to documents too.
The only time a surprise witness or document can be used at trial is for impeachment purposes. If a witness lies on the stand, then a party is allowed to bring in evidence that impeaches the credibility of that witness. But even then, the information is generally disclosed before trial because it would have been requested during discovery.
You can see also that it is vitally important to enforce proper discovery responses prior to trial. If you fail to conduct discovery, or if the opposing party just objects to your discovery requests and you do nothing about it, then they may be able to successfully hide a document or witness and spring it on you at time of trial.
In the end, it is up to you to enforce the rules so you can have a fair trial. And the more you play by the rules yourself, the more assured you are of being able to use the documents and witnesses that support your case at trial.