Preliminary hearings differ from trials in many important respects:
(For information on a related topic, see What is a probable cause hearing?)
After a preliminary hearing, prosecutors and defense attorneys sometimes agree to “submit the case on the record.” When this happens, a judge (not a jury) determines the defendant's guilt or innocence based on the judge's review of the preliminary hearing transcript. A prosecutor might agree to submit on the record when the case is weak but the prosecutor's office doesn't want to dismiss charges outright. If the judge dismisses the case, the prosecutor can deflect criticism from angry victims or police officers to the judge.
More often, a case submitted on the record favors the prosecution rather than the defense, and in essence is a slow plea of guilty. In such cases, the defense knows that a guilty verdict is all but certain, but by submitting on the record, the defense can move the case more quickly to an appellate court or simply offer an out to a defendant whose case is hopeless but doesn't want to plead guilty or nolo contendere. (Defense attorneys can submit on the record only if the defendant agrees to waive trial.)
Even though the defense doesn't expect to see all the prosecution's cards, the preliminary hearing may give the defense a preview of how strong the prosecution's tangible evidence is, how persuasive the prosecution's witnesses are, and how believable those witnesses are likely to be should the case go to trial. The defense tries to size up how solid the government's case is as a whole. Such information can be important to the defense, whether it ultimately settles the case in a plea bargain or proceeds to trial.
If the prosecution's case seems weak—if, for example, prosecution witnesses change their earlier stories, forget important details, or are otherwise discredited—the defense may decide it's worthwhile to proceed to trial. The prosecution, on the other hand, may be prompted to offer a generous deal, or at least the defense may gain leverage to push for one. If, however, the government's case seems very strong, this information may help the defense decide to accept a plea bargain, even if it's not what the defense had hoped for, rather than wasting further energy and money fighting what looks to be a losing battle. Because more than 90% of cases end before trial, it's clear that a primary defense goal at the preliminary hearing is to look for evidence it can use to get the best possible result at the plea bargaining table.
For example, if an arresting officer's credibility can be undermined during the preliminary hearing, and that officer is the state's main (or only) witness, the prosecutor may be willing to offer a much better deal following the prelim than the prosecutor would have if the officer had been a better (more believable) witness. Information gathered at the preliminary hearing will also help the defense if the case is one of the few that do go to trial. Whether or not the defense presents its own witnesses, the defense will usually vigorously cross-examine prosecution witnesses in the preliminary hearing. This cross-examination gives the defense an opportunity to see how the prosecution witnesses will hold up, and to pin them down as to what their testimony will be at trial. (If they change their testimony at trial, the preliminary hearing testimony can be used to attack, or "impeach" their credibility.)
As useful as a vigorous cross-examination of prosecution witnesses may be, a sound alternative defense strategy is to cross-examine prosecution witnesses very briefly and politely. This serves two purposes:
First, it may relax and lull a witness into admitting damaging evidence either then and there, or later when the defense attorney unexpectedly gets aggressive at trial.
Second, the defense may save evidence that hurts the witness's credibility and spring it on the witness at trial. Because the defense did not produce this evidence at the preliminary hearing, the witness may not be expecting it at the trial, and the surprise may fluster the witness and make him or her look bad in the eyes of the jury.