As a general principle, the prosecution is prohibited from offering evidence of an accused person's character at trial. If, however, the accused person offers evidence of his or her character first, then the state may do the same in rebuttal. In either instance, the trait of character may be proven up by reputation evidence only. Thus, where an accused person is charged with a theft or fraud type offense, he or she may want to offer evidence of his or her reputation for honesty. Where an accused person is charged with an violent offense, he or she may want to offer evidence of his her reputation for being a peaceful, non-violent person. This is usually done by calling witnesses who have known the accused person for some time and are familiar with that person's reputation for the the pertinent trait of character.
Specific instances of an accused person's prior crimes or bad acts is inadmissible at trial. In a DUI prosecution, for example, the state cannot offer evidence of accused person's prior DUIs for the purposes of establishing that he or she has the propensity to drive while intoxicated and, therefore, did so on the date in question. The underlying reasoning for this evidentiary rule is that a person should be judged for his or her present conduct and not for things that he or she may have done in the past. Just because a person drove while intoxicated three years ago does not mean that the person drove while intoxicated on this occasion. This exclusionary rule is based on principles of fundamental fairness.
The Williams' Rule, which is codified in Florida Rule of Evidence 90.404(2), constitutes an exception to this general principle. Here, evidence of other crimes, wrongs, or acts of the accused is admissible if it is relevant to some issue in the case, other than propensity to commit the crime. These purposes include proof of: (1) motive; (2) opportunity; (3) intent; (4) preparation; (5) plan; (6) knowledge; (7) identity; or (8) absence of mistake. William's Rule evidence may also be offered to rebut an entrapment defense (where the accused person asserts that he or she lacked predisposition to commit the crime) or an alibi defense (where the accused asserts, for example, that he or she was out of town at the time the charged offense was committed, and it can be shown that the accused committed another crime in this area around the time the charged offense was committed, evidence of the uncharged crime may be admissible).
With regard to motive, consider the following example. The defendant is charged with murdering his employer. There is evidence that the defendant had been stealing substantial sums of money from his employer over a long period of time, was caught, and was facing a lengthy prison sentence. The state would likely move to offer evidence of the theft - not show that the defendant is a bad guy but, rather, to show that the defendant had a motive to kill his employer (he was committing a theft, had been caught, and did not want to be prosecuted for it). With regard to plan or plan or preparation: assume the defendant is charged with bank robbery and there is evidence that the day before he shoplifted a handgun and a ski mask from a local Wal-Mart. Evidence of the previous day's activities would likely be admissible - not to show that the defendant has the propensity to steal things but because it shows how he or she planned and prepared for (and thus committed) the charged offense. With regard to knowledge: assume the defendant is charged with driving on a suspended license and claims he did not have knowledge of the suspension. Evidence of a recent prior conviction would likely be admissible to rebut the asserted lack of knowledge. The state would also likely introduce a driving record to show that the license had not been reinstated between the collateral offense and the charged offense. What about identification? Let's borrow an example from Hollywood. Remember The Silence of the Lambs where Buffalo Bill would place a unique, imported moth into the mouths of his homicide victims? If he was on trial for murdering one of the victims, and he raised the proverbial "SODDI" defense ("some other dude did it") evidence in the other homicides may be admissible to prove he is the killer because of the very unique modus operandi involved.
Where the state intends to offer this type of evidence, they must provide written notice to the defense prior to trial with a short and plain statement of its nature and the purpose for which it is being offered. Needless to say, this is almost always a contested issue. The defense will usually file what is called a "motion in limine" in an effort to keep this type of evidence from being admitted at trial. When it is admitted, the results can be utterly devastating to the defense.
The defendant need not have been charged with the prior act; that it was committed is enough. However, the state must establish its commission by "clear and convincing evidence". Evidence of the prior bad act(s) is usually presented to the court in a pre-trial hearing (typically after the defendant's motion in limine has been filed) so that the judge can determine whether (1) the state has met the requisite burden of proof; (2) whether the evidence is relevant to one of the narrow issues presented above; and (3) whether its relevance is not substantially outweighed by the danger of unfair prejudice to the accused. If the state does not meet its burden, if the court determines that the evidence is relevant only for the purpose of proving bad character, or the relevance is substantially outweighed by unfair prejudice, it will not allow the evidence to be presented to the jury at trial. Otherwise, the evidence will be admitted. The court will instruct the jury that it cannot consider the evidence for propensity to commit the crime charged, but only for the very narrow purpose it was admitted for. In an ideal world, this instruction would be effective. In reality, however, I think it is very difficult for a juror in a burglary case (for example) to hear evidence of prior burglaries committed by the defendant, and consider it for anything but propensity - instruction or not.
Florida has gradually, and increasingly, relaxed its standards for admissibility of prior bad acts in sexual battery and child molestation cases. In sexual battery cases, evidence of other purported sexual batteries is admissible to corroborate the testimony of the alleged victim in the pending case. This is in both familial and non-familial settings (familial is where all victims bear the same family relationship to the accused). There must be a showing, by the state, of uniqueness or similarity, which is even greater in non-familial situations. Of course, the relevance of the proffered evidence must not be substantially outweighed its prejudicial effect. In child molestation cases, evidence of prior acts of sexual molestation "may be considered for its bearing on any matter to which it is relevant". While it is not admissible to corroborate the victim's testimony (as with sexual battery cases), this verbiage significantly relaxes the general prohibition on admissibility of collateral crime evidence.
Williams Rule evidence can make a very mediocre case for the state a very strong case, very quickly. When a Williams Rule Notice is filed by the prosecution, every conceivable effort must be made by the defense to keep it from being admitted. Despite the court's instructions, and efforts by the defense to have the jury consider it only for its limited purpose, its relevance for proving propensity is the pink elephant in the room that can hardly be ignored. Effective motion drafting and argument can, however, go a long way in keeping collateral crime evidence where it belongs - in the past.