...Self-defense is all-or-nothing. In order to establish it, the client has to admit being at the crime scene, with a weapon, which he or she used to intentionally harm the aggressor. The client has to admit that he injured the aggressor. The client has to convince the jury that if a reasonable person had been standing in his shoes, the reasonable person would have done the same thing. In effect, the aggressor invited his fate by threatening or inflicting serious bodily harm, or by threatening to kill the client.
In one fell swoop, the client has given up alibi and mistaken identity defenses. He or she has given up any claim that the wound was made by accident. Generally, the client must give up provocation (heat of passion or extreme emotional disturbance). Logically, provocation implies an unreasonable response to a situation, and mitigates murder to manslaughter. Self-defense implies a rational response to a very dangerous situation and, if successful, results in an acquittal. Similarly, the client must give up claims of mental illness or insanity and defenses based on intoxication or drug use....
...
... Often, the defendant will need to testify in order to establish his subjective belief about the threat and need to respond defensively. This can be done through circumstantial evidence, but it is difficult....