This is an excerpt from one of my favorite expert on self defense, Rory Miller

"For a person to be seen as a valid and immediate threat, he must exhibits 3 elements: intent, means and opportunity.

*intent means he wants to hurt you
*means is his ability to hurt you: fist, knife, gun
*opportunity allow him to reach you with the means

If you can take one of these away he ceases to be a threat. If you can physically beat him up, you deny him his means. If you can run away, he is denied his opportunity. If you can change the threat's mind, you have altered his intent. Altering intent is a broad and potent skill. You can do it by projecting calm, bluffing, using humor, emitting an ear splitting scream, and a number of other ways."
-Rory Miller