A character who is not surprised has the option of making an Active Defense when attacked. A character can only be engaged in one active defense at the time; if you wish to make a second Active Defense while the bonus from a previous one still applies, you pay the price for the new active defense, and lose the bonuses from the old one.
A normal Active Defense is either a Parry or a Dodge, but the difference is a matter of style. You reduce your shot counter by one, and increase your Dodge value by +3 for the rest of the shot. You can do this at any time, regardless of what your current shot is, as log as you have shots remaining for the sequence.
Active defenses can apply to game values other than Dodge. An example is the defense allowed when you are being sneaked up upon.
There is a wide variety of Schticks, especially Fu, that introduce additional types of Active Defense. Not all of these have a shot cost.
Defending Others
You can actively defend another character, but this is a 3-shot action. You can do this with the various bonus defenses (Fu schticks and such), but pay three shots instead of the normal shot cost of the maneuver.
As an option, you can choose to interpose yourself between the attacker and his target. This voids the normal +3 Dodge bonus for an Active Defense, but redirects the attack to you. It is still a 3-shot stunt.
It is possible to make Snapshots while defending others.