Generally character evidence is avoided in most cases. Often, character evidence that is negative against an individual is excluded as unduly prejudicial. Trial

Generally character evidence is avoided in most cases. Often, character evidence that is negative against an individual is excluded as unduly prejudicial. Trials determine legal facts and judge whether these facts constitute a crime or liability (depending on whether a criminal or civil case). What a person is like is not a legal fact. Even so far as "being the type of person who has been convicted of a crime in the past" is not a legal fact in a case of "did this person do this new thing?" There are two common exceptions. The first is establishing the bona fides of an expert witness - you want to establish the individual as an expert in the eyes of the court. Only an expert may opine (offer an opinion) regarding facts to which they were not a witness. Lay witnesses may only testify to facts which they have observed. The second is with regard to truthfulness, as you have mentioned. If a witness's credibility can be called into question, then this can cast doubts on the veracity of their testimony. Having an established history of lying, or having perjured oneself in the past can be valid character evidence to cast doubt on testimony.