Fiction, by its nature, is in fact more rational and less random than reality. This is due to the fact that every aspect of the story is controlled by the writer who knows and plans for 'random' acts that are designed to further the story line. The 'random' freeway shooter putting holes in the hero's car (or in the hero) is on that bridge because the writer put him there. He is there because the writer needs him to be there pulling the trigger so the hero will be forced to go to the body shop (and maybe gaining a new love interest or a rental car) or be taken to the hospital (and getting personally involved in the police investigation into the shooter or maybe the writer just likes having the hero in the hospital). The point is, the 'random' acts in fiction should be anything but random.
Another truism: You can only write what you know. Whether it's romance, mysteries, thrillers, science-fiction or fantasy, the fictional world being created by the writer's mind must be internally consistent and have points of congruency with the world the rest of us are familiar with. Even if the work is one of fantasy with dragons and magic, there must be internal rules that are followed in that world. Whether explicitly stated or not, the writer must know and follow these internal rules. Nothing destroys the credibility of a piece of writing faster than the reader catching the writer ignoring the basic rules established for that work - it implies the writer didn't care enough about his own writing to 'do his homework'.
If the story is about flying saucers and aliens at Area 51, the writer had better know something about Groom Lake and the U.S. military in the real world. By establishing that the story is about aliens at Area 51, the writer has also established that the world being written about varies from the one the rest of us know about simply in the fact that there are aliens at Area 51. (Of course this does entail the differences in history from that world to this one, but since the subject is 'top-secret', the variations would naturally be limited to a small number of people 'in the know' in that world.) Aside from that critical difference, the rules of the 'real world' apply - Oak trees create acorns, smoking is bad for your health, Ford manufactures the Taurus, the McDonnell Phantom II is designated the F-4 with various letter designations. These things are ignored at the risk of alienating the reader - and an alienated reader will shy away from future works by the same writer.
Along with knowing the rules of the world being written about, there is also is related issue of "How does the character know what he knows?" While this does not appear to be as much an issue with mature writers (meaning over 30), this does crop up in writing by young people (college and younger) and in writing about young people.
It takes time to develop skills - according to researchers it take 10,000 hours of diligent work to develop true mastery of a skill. If the character is a teenager, has he had time to gain the skills he is shown having? If the character is a young street racer, and is still in school, has he had time to become a world class hacker, skier, shooter? Would he want those skills? And if those are necessary to the story, where did he learn and when? For that matter, if the character is a middle-aged bank executive, is he also a world class shooter, hacker, skier, mechanic, pilot? If so, when and where did he gain those skills and is there enough time in the day or week for him to remain proficient in those skills? And, equally importantly, does the mind set required to be a successful bank executive blend successfully with the mind sets generally found in people who have these other skills and if not, how does that dissonance impact the development of the character in question?
With this in mind, writers are always looking for ways to make their characters come alive, to make them more real, to get inside the character's 'head'. Thus, the Metaphysical Zone - Writing Fiction and Creating Fictional Characters.
One interesting method is to cast the horoscope of the character based on key personality elements being established by particular astrological aspects - preferably heavy planet aspects. By determining the approximate birth year of the character being studied (birth year as though he/she were a real person) and locating the dates within that time frame the particular aspect took place, a hypothetical birth date can be established for the casting of a natal horoscope.
One of the fascinating aspects of this process is how complex the character can become. Since no astrological aspect exists alone, using a key aspect to establish the chart brings with it all the varying, and sometimes contradictory, information one finds in the chart of the real person. These contradictions may well create a believable bank executive who spends his weekends flying his friends out to hunting and skiing when he's not working on his MG and is quite capable of hacking a main frame to find out who's been defrauding the company.
While the horoscope is an effective means of creating a complex character, there are other considerations as well. Numerology can help fine tune the character's name. Is he James, Jim, Jamos or Jamie to his friends and associates? Is he normally called by a title - Captain or Commander, for instance? These variations in what the character is called can reflect the differing personality aspects the character shows in various situations.
The personality type (Meyer-Briggs) and auric color are additional reflections of the personality of the character. The personality type can be derived from the horoscope and/or numerological study. The auric color is also a reflection of underlying personality and although I am not aware of research to confirm this in real people, I suspect the Sun/Moon signs would give a good indication of auric color unless the character has undergone a life-altering experience.