Character Evidence

Witness for the defense opens the door by offering good character evidence (reputation or opinion) of a trait pertinent to the crime for which defendant is tried. This is bad for the prosecution; the jury has just heard something good about the defendant. The jury might like the defendant now! What can the prosecution do? There are two options, and it's important to keep these options separate because they aren't identical. ~The prosecution might choose to cross examine the character witness. If it does that, it's not bound by reputation evidence or opinion evidence. It can offer specific instances of conduct to rebut the witness's prior testimony. ~The prosecution might choose to call up its own witness on direct to rebut the defendant's character witness. In that case, though, it'll be bound by reputation or opinion evidence. Why the difference? For a bar exam, the most important thing is to remember that there is a difference. But, at least for me, the "why" behind legal rules has been the way I've learned law.  

When cross examining the character witness, it's not only about the character of defendant; it's also about the credibility of the character witness. And it's entirely proper to impeach that character witness by asking about these types of specific acts.