Defenses are arguments with supporting evidence that a defense attorney puts forth to secure the freedom of his or her client. A defense grows out of a defendant's version of the events in the alleged crime. It is intended to bring about the most favorable outcome for the defendant (for example, a verdict of not guilty or an acceptable plea agreement).
When developing a defense strategy, the defense attorney considers the credibility of defense and prosecution witnesses, community attitudes toward the crime and the defendant, and the nature of the prosecution's evidence. An important ethical rule is that a defense attorney can't knowingly encourage or help a defendant lie under oath (in other words, commit perjury) and consequently is forbidden to call a witness who he or she knows will lie on the witness stand. Even if a defense attorney knows his or her client is guilty, the attorney can cross‐examine prosecution witnesses and poke holes in the prosecution's case. This procedure is permissible because it is the defense attorney's responsibility to make the prosecution prove its case.
The most common defense
The most common defense is that the prosecution failed to prove the defendant guilty beyond a reasonable doubt. By raising questions about the credibility of the prosecution's witnesses, the defense counsel seeks to create reasonable doubt in the minds of the jurors so they will acquit the defendant. To cast doubt on the truthfulness and reliability of prosecution witnesses, a defense attorney can use any or all of these tactics:
Demonstrate bias on the part of prosecution witnesses, who, therefore, may be lying.
Expose police mistakes in gathering, maintaining, and testing physical evidence.
Suggest that the prosecution has bribed a witness by granting immunity from prosecution for pending criminal charges in exchange for the witness's testimony against the defendant.
Challenge the believability of a witness's story on the grounds of logic or common sense.
Mounting effective challenges to various aspects of the prosecution's case costs money. Money buys resources—investigators, forensic scientists, and legal experts—that can influence the outcome in a trial. O. J. Simpson's expenditures of $6 million on his defense, for example, may have bought him an acquittal.