In addition to creating realistic characters, in realistic (if not fantastic) situations, I've included a few exercises to help build those literary muscles. At the end of this piece, there should be some interesting situations
Once you believe you have your cast of characters in full dimension, it is up to you to test these characters to insure they are not merely tailored for a single situation. Any real life human being can train to work in any line of industry. They can learn any trade. They can do basically whatever they put their mind to.
Your characters are no different. Your characters WANT to exist in your work... even if they eventually have to die, while they exist they want a FULL existence. You can provide this for them by avoiding the tailor-made characters. If your story is a crime drama, and Detective John Doe is the lead character, he had better know more than just detective work. Understand that as "real" people, your character had a life before law, or crime, or super-heroism, horror, or whatever situation you put them into. The detective may have a Honda now, but he didn't when he was sixteen... or maybe he did, but now he's 36, and its the same car. Understand that Detective John Doe first started out the workforce at 16, flipping burgers for Happy John's Burger Basket. Or He was an usher for Leroy's Theaters. Maybe he was a mop boy for some gym... perhaps detective work was his hobby back then. Perhaps he wanted to be a magician, and the attention to detail required lead him into his line of work as a detective. My point is: He's not 100% pure detective.
Tailoring characters to a story, unless it's a fairy tale, or story with a "more of the story is:..." will insult your audience. People who cannot relate to your characters will forget them.
To help you put your characters to the test, I will provide a distinct list of challenges I call the gauntlet. Take your best characters, male, or female: BOTH good, and evil and place them through the gauntlet. IF your characters are tailored, this will help flesh them out; if they're full characters, this will be a fun exercise to see how they respond. Remember: Have fun.
Good luck.
The Gauntlet (character prompts by Cassus Finley).
CHALLENGE ONE: Protagonist (M/F/) / Antagonist (M/F)
The Gas Station.
Work is incredibly difficult for your hero/villain. The market's down, and the economy is a bust. As such, the Hero Dept/Villains Inc. has closed down and forced your protagonist / antagonist to seek new work.