Form interrogatories look innocent enough. Drafted by the Judicial Council, these questions usually ask parties in civil actions for basic information: What are the facts? Who are the witnesses? What are the documents? These simple inquiries often require a painstaking effort - and lots of time - to answer. Some attorneys go to great lengths to avoid providing straight answers. But form interrogatories are important and should never be treated lightly. Consider Form Interrogatory 15.1, the bane of the defense bar. It requires defendants to identify all facts, witnesses, and documents in support of, among other things, special or affirmative defenses they have raised in the pleadings. When cases are in their early stages, defendants often object to answering Form Interrogatory 15.1 on the ground that the question is premature. They may contend that they pleaded various affirmative defenses simply to preserve the right to assert them later, after supporting evidence emerges from the fog of discovery. But beware of a dangerous trap for the unwary: If a defendant initially objects and later also fails to supply facts in support of an affirmative defense in response to supplemental discovery, the plaintiff may file a motion in limine on the eve of trial to bar the defense from proving its case. If granted, the motion can be devastating. Motions in limine are a commonly used tool of trial advocacy in both criminal and civil cases. They usually occur at the outset of a trial, such as during a preliminary conference with the assigned judge. But in limine motions may also arise during trial, to deal with unanticipated evidentiary issues. The primary purpose of these motions is to preclude the introduction of inadmissible and prejudicial evidence. (See Kelly v. New West Federal Savings, 49 Cal. App. 4th 659, 670 (1996).) A typical in limine order designates evidence for exclusion, and it prohibits counsel, parties, and witnesses from referring to the excluded material during trial. These motions are designed to prevent problems before they arise; every experienced trial lawyer knows it's almost impossible to "unring the bell." It's far better to anticipate the problem. Motions in limine also serve other purposes: They minimize sidebar conferences, thus allowing for an uninterrupted flow of evidence; and they can truncate a party's case, thereby promoting a greater possibility of settlement. California Evidence Code section 352 gives the trial judge discretion to exclude evidence when its probative value is substantially outweighed by the likelihood that admitting it would take up undue time, or create a substantial danger of prejudice, confuse the issues, or mislead the jury. However, this discretion is not absolute. The court must exercise it reasonably in accordance with the facts (Brainard v. Cotner, 59 Cal. App. 3d 790, 796 (1976)). Moreover, "[w]here the evidence relates to a critical issue, directly supports an inference relevant to that issue, and other evidence does not as directly support the same inference, the testimony must be received over a section 352 objection absent highly unusual circumstances." (Kessler v. Gray, 77 Cal. App. 3d 284, 291 (1978).) Many courts proscribe utilizing motions in limine for purposes other than evidence exclusion. In Los Angeles County for example, local rule 8.92 (b) prohibits a party from seeking summary judgment via a motion in limine. The rule directs that such motions may be brought only in compliance with the summary judgment statute (Cal. Code Civ. Proc. § 437c). Likewise, the Los Angeles rule instructs that motions in limine cannot juggle the order of proof, which is controlled by section 598 of the Code of Civil Procedure. But a motion in limine can be effectively used to exclude facts, witnesses, and documents supporting affirmative defenses. Such motions can eviscerate an opponent's case, and they do not run afoul of other rules concerning dispositive motions. As for timing, there's a reason a party makes a motion in limine on the eve of trial - after the opposition has had ample time for investigation and discovery. In this setting, a defendant cannot credibly argue there wasn't sufficient opportunity to research the merits of a defense. The time has come to fish or cut bait. When you receive form interrogatories, take them seriously. It is crucial for a defendant to provide a substantive response, whether initially or in response to supplemental discovery. Failure to do so brings the risk that a motion in limine will eradicate a potentially successful defense when the day of trial arrives. Craig A. Roeb is chair of the business, employment, and product liability practice groups at Chapman, Glucksman, Dean, Roeb & Barger in Los Angeles. Law clerk David Napper assisted with this article.