Judges are, in many ways, referees. They act as gatekeepers for evidence, deciding whether it comes in. But judges don’t often act on their own. Rather, the attorneys trying the case must force the issue.
The defense can’t assume that a judge will enforce the evidence rules without objections. Unlike football and basketball referees, who call fouls without waiting for the fouled team to make a request, judges don’t strike improper evidence on their own volition that often. They usually rule only when the defense or prosecution asks for a ruling through an objection (or, sometimes, through an affirmative request that a judge admit evidence). To object during testimony, all the defense or prosecutor must do is succinctly point out the reason the testimony is inadmissible.
For example, if the prosecutor asks a police officer to testify to a statement made by a witness shortly after the commission of a crime, the defense might say, “Objection, hearsay.” If the judge finds that the objection is proper, the objection will be sustained. Otherwise, the judge will overrule it, or perhaps ask for clarification regarding the objection.
Sometimes attorneys let witnesses give inadmissible testimony without objecting to it. But that doesn’t always mean these lawyers are acting incompetently. Making objections is as much an art as a science. Even if prosecution evidence is inadmissible, a defense attorney may decide not to object for any number of reasons. For example, the attorney might think:
Whatever their reasons, defense attorneys typically have to make instantaneous decisions about whether to object. It’s relatively rare that their failures to object will create a viable claim for ineffective assistance of counsel. There are, however, instances in which it will. Take, for example, a lawyer’s lack of objection to testimony about the defendant purportedly belonging to a criminal organization—that's the kind of omission that might eventually lead to a new trial. (See, for example, Mitchell v. State, 298 S.C. 186 (1989).)
If you’re facing criminal charges, you’re almost always best off letting a lawyer represent you. The rules of evidence are tremendously complex and nuanced; only an experienced attorney will be familiar enough with them to provide a competent defense to criminal charges.