Evidence about prior criminal offenses is generally not admissible in court, unless it is directly related to the current charge. Every United States citizen is entitled to a fair trial, so it is important to avoid the inference that the defendant committed the crime simply because he or she is a person of criminal character. That is why evidence of other crimes is inadmissible unless it has an independent relevancy besides showing a criminal disposition.
So for example, if you are charged with felony shoplifting, a videotape of you previously shoplifting would be admissible, but a video of you committing an unrelated crime – such as trespassing – would not be admissible. Trespassing has no relevance to the charge at hand – i.e. your guilt or innocence of the shoplifting charge. The trespassing evidence might suggest that you have criminal tendencies, but it is not admissible in trial since it does not necessarily mean that you committed this particular crime of shoplifting.
However, evidence of other unrelated crimes can be admissible under certain circumstances. A prior conviction could be considered relevant if the way the previous crime was committed is consistent with the way the current crime is committed. This would show a pattern of behavior and could be admissible evidence. The district attorney must notify the defendant and the criminal defense lawyer first if they intend to introduce evidence of previous crimes.
I cannot emphasize how important it is to have a skilled criminal defense attorney at your trial – an experienced defense lawyer will be aware of these evidentiary rules and use them to protect your constitutional right to a fair trial.