In addition to using court discovery procedures to obtain evidence from the prosecution, defense attorneys have a duty to investigate their clients' cases. Effective lawyers will gather evidence of their own in preparation for trial—and even to see whether the client has a reasonable chance of winning at trial.
Defense investigation methods can be as informal as talking to potential witnesses on the telephone or as formal as serving a cellphone company with a subpoena demanding call logs. One method that's often effective is in-person interviews of those who know about either the events underlying the charges or the people involved in the case. This includes interviewing even those who might testify for the prosecution.
The defense can gain significant benefits from trying to interview prosecution witnesses rather than relying on their statements. These include the ability to:
Some defendants might wonder whether it's worth it to interview a witness who has already given a statement that the prosecution has disclosed. But prosecutors and police officers sometimes omit or misstate information (either intentionally or not). Further, when law enforcement and prosecutors speak to witnesses, they aren't likely to ask all the questions the answers to which the defense would like to know. And there might not be a record of all conversations witnesses have had with the other side.
It's perfectly legal for defense attorneys and their investigators to interview prosecution witnesses in most instances. (Among the instances in which it's not are those involving harassment or threats.) And even though prosecutors might not want their witnesses—including police officers and victims—to talk to the defense, they typically can't stop them (though they may "inform" them that they don't need to).
It's generally up to witnesses and victims to decide whether to talk to the defense before trial. They might not be willing to talk, but a defense attorney or investigator who doesn't ask often doesn't know.
The defense has limited options for trying to interview uncooperative prosecution witnesses before trial. The defense lawyer might hire a private investigator who specializes in finding and interviewing reluctant witnesses. (A defense attorney might be capable of persuading a hesitant witness to talk, but will typically prefer to have someone else try, or at least be present during the conversation—that way the lawyer doesn't have to testify if the witness says something new or inconsistent at trial.)
Private investigators can drive up the cost of a case, but they're often worth the money. For those defendants who can't afford investigators, or even attorneys, the court might in some instances (usually serious cases) appoint a free-of-charge investigator. Public defenders' offices typically have investigators who work on various clients' cases, but their workload often limits what they can do.
In some jurisdictions, the defense can serve witnesses with subpoenas that compel them to answer questions at a deposition (an out-of-court session at which the witness may be questioned under oath). Depositions are common in civil cases, but far less frequent in criminal cases: Most jurisdictions don't allow criminal depositions.
If you're facing criminal charges, consult an experienced criminal defense lawyer. Only such a lawyer can protect your rights and effectively investigate your case. An attorney will also be able to tell you what you should—and shouldn't—do to help. For example, your lawyer may ask you to provide all the information you can possibly think of regarding witnesses, locations, events, and even people who have nice things to say about you (who can be relevant not only at trial, but also at potential sentencing).