How does one introduce evidence into trial? Do all pieces of evidence have to be introduced into trial before trial starts? Can I bring additional evidence on the day of the trial and introduce it on the fly to the judge when I make my arguments? Why or why not?

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How does one introduce evidence into trial? Do all evidences have to be introduce into trial before trial starts? Can I bring evidences on the day of the trial and introduce them on the fly to the judge when I make my arguments?

In general

The rules vary from court to court with the exact rules spelled out in general court rules for the particular kind of court in question, local rules of that particular court and standing orders of the presiding judge, and case specific case management and trial management orders of the judge.

Generally, less advanced notice is needed in courts of limited jurisdiction (e.g. limited to claims of $15,000 of less for money damages or misdemeanors only), and more notice is required in courts of general jurisdiction (which have authority over almost any civil matter or crime, subject to federal v. state court divisions). There are also differences between civil cases and criminal cases. These are discussed briefly at the end of this answer.

Evidentiary hearings on pre-trial matters (e.g. preliminary injunctions in civil cases, or evidence suppression hearings in criminal cases) are typically conducted with a process involving less pre-trial disclosure that is more similar to trials in limited jurisdiction courts.

Civil cases in general jurisdiction courts

Before trial

In a typical U.S. civil case in a court of general jurisdiction, the exact deadlines vary, but the existence of documents relevant to the case and of people with knowledge of the facts who could testify in the case has to be disclosed fairly early on in the court case, or in connection with "discovery" (such as depositions and written requests for documents) and expert witness disclosures in the middle of the case.

Then, there is typically a duty to disclose a trial witness list and proposed trial exhibits a few weeks before trial to the other parties. Detailed disclosures of what witnesses will testify to is typically not required, but there will have usually been an opportunity to take depositions of, or try to interview, witnesses during the discovery portion of the case in the middle of the pretrial process.

A party can object to anticipated exhibits or witness testimony before trial in what is called a "motion in limine" and sometimes a judge will resolve the question of whether that evidence may or may not be admitted in advance, and at other times, the judge will reserve ruling on that question until the question soliciting witness testimony is asked, or the exhibits is presented at trial.

At Trial

Once this has been done, when the trial actually commences, stipulated exhibits are often admitted immediately, and then you call witnesses when it is your turn to do so (plaintiffs go first, then defendants, then plaintiffs can bring rebuttal witnesses).

Non-stipulated exhibits (whose admissibility has not been ruled upon before the trial begins in connection with a motion in limine) are typically offered in connection with witness testimony one by one. When such an exhibit is offered into evidence, this is subject to evidence objections raised by the other side at the time that the exhibit is offered. If there is an evidence objection during the trial, the judge rules on the objection on the spot, either overruling it and admitted the exhibit into evidence, or sustaining the objection and not allowing the exhibit to be admitted. Objections are resolved based upon the applicable rules of evidence and a determination regarding whether it was properly disclosed prior to the trial.

Certain kinds of evidence (either witnesses or exhibits), such as "impeachment" and "rebuttal" evidence (i.e. evidence to show that testimony was false or that an exhibit shouldn't be taken seriously), can be admitted without prior disclosure, but other kinds of evidence needed to be disclosed in advance to be admissible at trial.

The rules of evidence about what can or cannot be admitted at trial are typically the subject of a one semester law school course and are beyond the scope of an open ended question asking the very basics like this one. They include rules about topics such as character evidence, relevance, evidentiary privileges, expert witness and opinion testimony, hearsay, and authenticity objections. The federal rules of evidence are typically and form a model for state rules of evidence in many states, although a few states like New York and California have their own rules of evidence that are structured differently but similar in substance.

Limited jurisdiction courts and criminal cases

Some courts of limited jurisdiction will have no pretrial disclosure requirements and no discovery. Others will require, at least, a disclosure of the exhibits to be used at trial and the witnesses who will testify at trial, and typically dispense with motion in limine practice.

While disclosure and discovery requirements in civil cases typically treat all parties equally, in a criminal case, the prosecution has very extensive pretrial disclosure and discovery obligations, while criminal defendants typically have only minimal pretrial disclosure and discovery obligations. Criminal defendants do, however, typically have to disclose in advance if they plan to make an alibi defense and if they plan to make an insanity defense.

Why or why not?

Evidence rules try to strike a balance between a "trial by surprise" that could cause the outcome of the trial to be due to something that the other side didn't have an opportunity to get evidence to respond to, and the cost of making pre-trial disclosures or discovery.

This balance is struck differently depending upon what is at stake. With lower stakes, accuracy is less important and cost is a bigger concern. With higher stakes, accuracy matters more and cost is less of a concern. In criminal cases, the 5th Amendment right not to self-incriminate yourself limits the discovery obligations of criminal defendants.

Common Law v. Civil Law Legal Systems Compared

Towering over this is the extreme level of finality that comes with trial results in common law system court cases. After the trial is over, there is almost no room for "do overs" or supplements to the evidence or make new legal arguments.

In contrast, in the civil law systems, for example of continental Europe in places like France and Germany and Italy and Spain, after a first trial is completed in a case, on a direct appeal of that decision new evidence as well as new legal arguments can be considered by a larger panel of more senior judges. So, getting it right the first time despite potential surprises in the first trial is not as much of an imperative.

The other big difference between common law and civil law trials is that common law trial procedures were designed around the possibility of a jury trial (even when cases are actually tried before a judge without a jury) with all evidence admitted in a single more or less continuous trial.

In contrast, civil law trials are before judges and often receive evidence piecemeal with one or a few witnesses testifying at each session and evidence taking sessions spread out over weeks or months, something that is possible because a small panel of full time judges can be reconvened much more easily than a common law jury.

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