Rules of evidence are the trial system’s equivalent of the rules of grammar. Just as grammar rules govern how we speak and write, evidence rules control courtroom procedures. Evidence rules limit not only what witnesses and lawyers can say during trials, but also how they can say it.
The same evidence rules apply to both judge and jury trials. Most jurisdictions have only one set of evidence rules, applicable generally to all trials. In practice, however, judges tend to apply evidence rules much less strictly in cases tried before a judge than in those before a jury.
Relevance is the basic building block of evidence rules—evidence must be relevant to be admissible. For evidence to be relevant, there must be some logical connection between it and the fact it’s offered to prove or disprove.
Witnesses in both criminal and civil cases can’t testify about something they don’t personally know about. The “personal knowledge” rule (Federal Rule of Evidence 602) requires that all non-expert witnesses testify based on firsthand information. It’s not enough that they got the information secondhand (as is the case with hearsay), even from a reliable source. Nor can they testify about information they have good reason to believe, but can only assume is true.
When they're depicted on television, prosecutors and other lawyers are often aggressive and seem to make statements to, rather than ask questions of, witnesses. Is this how it goes in a real courtroom?
In television dramas, closing arguments are the height of the trial: The prosecutor and defense lawyer each deliver an emotional plea for justice. In real life, closing arguments are a way for the attorneys to pull together all the evidence for the jury—they’re intended to appeal to jurors’ reason,
Judges are, in many ways, referees. In jury trials, they act as gatekeepers for evidence—they decide whether it comes in or not. But judges don’t often act on their own. Rather, the attorneys trying the case must force the issue.
Potential defense witnesses in criminal trials (defendants included) might wonder whether defense attorneys can guide their testimony. The answer is yes and no. Lawyers can certainly discuss testimony with witnesses in advance of actually testifying, but they can’t direct them as to what, exactly, to say.
Experts have special education, training, or experience that allows them to testify to their opinions on matters beyond everyday understanding. They aren’t bound by the personal knowledge rule, meaning that they need not have directly observed the facts underlying the case.
The “mercy rule” allows a criminal defendant to offer evidence of his or her good character as a defense to criminal charges. (Federal Rule of Evidence.) Evidence of good character isn’t allowed for the purpose of arguing that the defendant committed the crime, but shouldn’t be convicted. Rather, it’s admissible to show that the defendant is unlikely to have committed the alleged crime(s).