Rule 787 Subject to Section 788, evidence of specific instances of his conduct relevant only as tending to prove a trait of his character is inadmissible to at…

When someone is a witness in a case, the other side cannot bring up specific things that person has done in the past just to make them look like a good or bad person. For example, if someone once lied to a friend years ago, that story cannot be used just to suggest the witness is generally a dishonest person.

There is an exception to this rule, which is covered in Section 788. That section deals with felony convictions, which can sometimes be used to challenge a witness's credibility under separate rules.