In some juridsdictions this is also known as motive. A friend who takes out the latest addition to his knife collection to show to you has both abiliy and opportunity. What is lacking is the reasonable perception that he is placing you in jeopardy by showing you his latest acquisition. As in the question of how far away is an assailant with a contact weapon still a threat, prior knowledge is a key element in judging your reasonableness. If you try to cite knowledge you didn't acquire until after the incident, it will not be allowed in court.
Documented training can make the difference.
Ability, opportunity and jeopardy form a tripod. If all three are not present simultaneously, you cannot prove justifiable use of force. I'm sure you've seen at least one Western movie where the evil gunfighter forces the innocent rancher to go for his gun. When the rancher is beaten to the draw, all the bad guy's buddies swear to the sheriff that the rancher went for his gun first. If you go armed in society and seek arguments and fights, you may find a judge or jury ruling that you gave up your mantle of innocence.