Lights, camera, action
It's a sad fact that few literary critics want to admit: If you want your work to be read by the masses, almost by definition it must have action in it. Therefore, the ability to write action-packed scenes is an essential skill in every writer's toolbox.
Perhaps the key quality of good action fiction is that it is compelling. It not only holds a reader's interest, but also rivets him so thoroughly that he is actually resentful at the idea of having his reading interrupted. It is the writer's duty to totally enmesh his reader, and the more thoroughly he succeeds in doing so, the more effective the action sequence will be.
As is almost always the case, the key to engaging a reader is the use of emotion. In the case of most action sequences, the emotions engaged will be suspense and fear. These are very intense and direct emotions, which (I've always believed) is why action sequences are often so memorable and vivid in the reader's mind. Consider, for a moment, perhaps the greatest action sequence ever filmed, the introduction to the James Bond movie Moonraker. The movie opens with our protagonist in an airplane, fighting it out with his antagonists. During the course of the fight, Bond is left alone with no parachute in an airplane that is clearly about to crash. Yet Bond jumps out anyway, and with this act utterly rivets the filmgoer. What in the world? the viewer asks himself. Why, Bond just killed himself for sure! Our hero then controls his fall in an all-or-nothing gamble in such a way as to collide in mid-air with one of his antagonists, who in fact does have a parachute. A gritty fight scene follows, in which both characters try to take control of the single parachute and thus save their life, flailing about nearly helplessly in mid-air without any solid ground to gain purchase on. Finally, of course, Bond manages to liberate the 'chute from the bad guy, after which he puts it on and -- at the last possible second -- pulls the ripcord .
There's a lot to be learned from this sequence, if one takes a few moments to analyze it. One thing that is visible right off is the sheer daring and vision of the writer that undertook to script such a scene. On the face of it, after all, not only is the basic premise a little ridiculous (after all, it takes time to unstrap and restrap a parachute, especially from someone who is resisting), but it borders on the corny. Yet the scene worked, and worked very well indeed. The lesson here is that an action writer needs to be daring, needs to be able to confront the ridiculous and yet at the same time make it seem to be deadly serious. This is not so difficult as it at first may seem, especially when one realizes how often the ridiculous intrudes into deadly serious situations in real life.
Another lesson to be learned from this classic bit of film is that oftentimes the best action sequences are written in settings that are unfamiliar and unconventional. Even back when Moonraker first aired, Bond had probably already defeated a hundred bad guys using his fists alone. Yet he had never done so in mid-air before, with the wind rushing past him and the ground rising up to kill him like a videotape on fast-forward. The average person has never had to think about the difficulties of hand-to-hand combat under such ridiculous conditions; punches are weakened, judo will not work, karate-kicks are useless. This means that the reader/viewer is covering fresh ground, and intrinsically cannot know what to expect next. This greatly aids the writer in building a sense of suspense.
Also vital to this scene is its sense of continual desperate improvisation. Not only did Bond have to physically defeat his enemies, he had to outthink them through an entire series of impossible situations as well. The sheer brilliance and daring of Bond's plan is a major part of the appeal of this scene, and the fact that he had to come up with it on the spur of the moment merely adds to the success of it all. I personally am very taken with this method of inducing suspense, and try to take advantage of it at every opportunity. One trick that I often employ is for my protagonist to develop some sort of plan to get out of a tight spot. This plan must be both credible and believable to the reader, so that he perceives it as the way out. Then I have the first few steps of the plan go perfectly, until something totally unforeseen and unforeseeable goes wrong, preferably leaving the protagonist standing naked in the center of a well-manicured football field, with about ten thousand bad guys armed with machine guns shooting at him. From this point the protagonist must improvise, improvise quickly, and improvise well. The totally unexpected development is a surprisingly effective technique for getting readers hooked.
Finally, the wordsmithing of action-writing is special, and deserving of mention. While an action scene does not by definition demand physical exertion, it does seem to require a sense of excitement and tension. The human body exhibits very definite and specific reactions to this kind of stress; the heart rate increases, breathing speeds up, and details of the environment seem to jump out and demand attention. By mimicking these physiological reactions in prose, the writer can convey an almost subliminal sense of danger that creates considerable emotional impact. Speeches should be short and direct, as if the speaker was short on breath. Similarly, sentences should be clipped and commas (which slow things down, exactly the opposite of what you want) avoided whenever possible. Beginning writers tend to write in overly-long sentences to begin with; in action scenes, a fair proportion of sentences should be of four words or less. Another facet of this phenomenon is paragraph length; in action sequences, more than anywhere else, some paragraphs ought to consist of a single, flat, declarative sentence.
Keep it short, keep it fast, keep it unpredictable; those are my rules of thumb when I sit down to write action. These guidelines have served me well. May they bring just as much success to your literary endeavors.