The principle behind mutual discovery was the realization that “Perry Mason”-type springing of evidence/witnesses on the other party at trial really did not further the ends of justice. Justice is best served by a full and fair hearing of all the evidence, with both parties having sufficient time beforehand to see, evaluate, and prepare for the evidence and/or witnesses.
I’m not a criminal attorney so I can’t speak to criminal law, but on the civil side of it, we are required to participate in discovery. Discovery is the period of time prior to trial but after the case is filed, in which both parties may serve on each other:
– interrogatories (written questions requiring written answers);
– requests for production (demands for paper, digital, or other records that might be relevant, including, say, medical records, or the actual auto part that is asserted to have caused an auto accident);
– requests for admissions (written demands that the other party admit or deny the truth of some salient point, i.e.: “Admit you were at the corner of First and Maple at 10 p.m. on the date of the accident”). If the other party admits to an RFA, he or she will be bound by that admission at trial.
A party may also take depositions, which is the in-person questioning of a witness by the attorney for the opposing side, with counsel for all parties usually present. A deposition is taken under oath, and recorded on paper by a court reporter (who will produce a deposition transcript) and in some cases is also recorded on audio or video tape.
Failure to cooperate with the other party’s discovery, if that discovery is proper and allowed under the rules, can lead to motions to compel discovery. A continued failure to cooperate or comply in extreme cases may result in the evidence being barred or even in the entire case being dismissed. IME, judges hate discovery disputes, and it is always in the best interests of the parties for counsel to work things out between themselves without involving the court at the discovery stage, if at all possible.
If the attorneys have done discovery correctly, they will have asked for any potential document, any possible witness, any even remotely relevant thing. This should prevent the other party from having any evidence in his/her back pocket to spring at trial.
But beyond that, modern civil trial practice generally (always, IME) requires the attorneys before trial to prepare witness lists and exhibit lists and to furnish them to the court AND to the other party. These lists are to include every witness you intend to call, and everything you intend to use as an exhibit. IME, parties are NOT generally required to disclose a list of everything they might try to enter as evidence, but through the discovery process, the witness lists, and the exhibit lists, the other party should have a pretty solid idea of what the evidence will be, or even could possibly be.
With these rules in place to provide for FULL disclosure of all potential evidence, a party who attempts to spring a new (that is, not previously disclosed) witness or piece of evidence on the opposing party at trial will very likely find the evidence/witness excluded, unless the party can provide a good reason why the evidence/witness wasn’t disclosed earlier. The most common reason for that is that the evidence/witness was only “newly discovered,” and therefore couldn’t be disclosed sooner. But judges look pretty hard at whether the evidence really was newly discovered, and even if it was, will generally give the opposing side some time (maybe a day or two) to evaluate asnd prepare for the newly discovered evidence. This may result in the trial being suspended for that time, a huge inconvenience to the court, the parties, and the jury, and so judges REALLY do not like newly discovered evidence.
Anyway, the attorneys taught to practice under the old system – don’t show the opposing party anything, try to smuggle stuff in to trial – are mostly retired, since the rules for full disclosure and extensive discovery have been in place since, IIRC, the early to mid-70s. But those old guys really didn’t like the new system, because they were taught to think of a trial as two attorneys trying to outwit each other, not necessarily as a complete revelation of what truly happened. The new rules of evidence and discovery therefore presented some serious difficulties to some of the old guard. Now, however, many attorneys have never practiced under any other rules (I haven’t), and even those who have, have been under the “new” rules for 20+ years.