One of the oldest instincts in a courtroom is to argue that a person did the thing because he is the kind of person who does that thing. It is also one of the things the law most distrusts, and for good reason. The rules on character evidence are a wall built on purpose, to keep a jury from condemning someone for who he is instead of what he did.
The basic rule, in Texas as in the federal courts, is that evidence of a person's character is not admissible to prove that he acted in keeping with it on a particular occasion. Texas Rule of Evidence 404 states it plainly: you generally cannot use a person's character, or a specific past act, to show he had a propensity to act that way and therefore did. The fact that a man has a temper is not proof he threw the punch. The fact that a driver has old tickets is not proof he was speeding this time. The law makes you prove what happened, not what the person is like.
The reason is that character evidence is powerful and unfair in equal measure. Told that a defendant has done bad things before, a jury tends to leap, to decide he is a bad person and therefore probably guilty, and to relax the state's real burden without noticing. It punishes the person for his record instead of for the proof. The rule exists precisely because the argument works too well on human nature, and works whether or not the person did this particular thing. So the law walls it off, to keep the trial about the event and not about the reputation.
Like every strong rule it has doors. Character can come in when it is genuinely at issue, and past acts can be admitted not to show propensity but for another real purpose, to prove motive, opportunity, intent, plan, knowledge, identity, or absence of mistake. That is the heart of Rule 404(b), and it is where these fights are won and lost, because the same piece of evidence can be forbidden propensity to one side and permitted proof of intent to the other. The skill is in the framing, and a careful judge polices the line hard, because the risk of misuse is always right there.
And when character does properly come in, the law limits how you may prove it. Under Rule 405 you generally do it through reputation or opinion, not by parading in a long list of specific incidents, with specific acts allowed only in the narrower situations the rule marks out. It is a deliberate throttle. Even when the door opens, the law lets through the general picture and keeps out the pile-on of detail that would turn the trial into a referendum on the person's whole life.
For the trial lawyer the work is on both sides of the wall. When the other side reaches for character dressed up as something else, you make them say out loud what it actually proves, and if the honest answer is only propensity, you keep it out. When you have a past act that genuinely proves intent or plan or identity, you frame it as exactly that, tie it to the permitted purpose, and ask for the limiting instruction that keeps it in bounds. The whole contest is over which side of the line the evidence falls on, and that contest is often the case.
So respect the word. The rules on character are the law insisting that a person be judged for what he did, on the proof of this event, and not condemned in advance for the kind of person someone says he is. It is one of the quiet guarantees of a fair trial, and defending it, or testing it, is some of the most important work a lawyer does before the jury ever hears a word.
See Texas Rule of Evidence 404 (character evidence is generally not admissible to prove conduct in conformity with it; evidence of other crimes, wrongs, or acts may be admissible for non-propensity purposes such as motive, opportunity, intent, plan, knowledge, identity, or absence of mistake) and Texas Rule of Evidence 405 (methods of proving character, chiefly by reputation or opinion). This is general information, not legal advice.
