My idea is a two sided tale told from the mind of two main characters. The main characters are best friends from childhood, but split at high school, take dramatic turns in opposite directions. In youth they had both dreamed of riches and wealth, but one character meets a girl and stops short of riches to 'settle' for a family, and a quiet suburban existance in their childhood town. The other becomes a cut-throat business man and play-boy. The story builds through their lives until they face each other approximately 20 years after high school and realize all the things they had not wished for weren't exactly all they were cracked up to be.

Damon Knight used to say that the story should start when things begin to happen.

So the first thing you need to figure out is what brings these two back together (since the story is about them facing each other), and what makes them rethink their goals?

OSC's M-I-C-E structures (discussed in HOW TO WRITE SCIENCE FICTION AND FANTASY) can help you with the rest of the structure, once you decide which kind of structure fits the story you want to tell.

I'm guessing you're thinking of a C=character story because your characters are questioning their roles (see the part on this in OSC's book).

Do you have them try a switch ("walk a mile in my shoes") so they can see how the other one lives--either mundanely or magically--or do you have something else in mind?

The story will end when they either change their roles or come to accept the roles they chose in the first place.

There was a movie (can't recall the title--something about a kiss) with Meg Ryan and Alec Baldwin about a bride who switches bodies with an old man, and how they eventually decide to switch back. This would be a variation on that idea.

If you don't have them actually switch roles, you need to find some other way that they can explore each other's roles and come to a decision about what kind of role they really want.

As for their characterization, ask yourself questions about each of them, maybe find a character questionaire to fill out for each of them, use astrological signs, pick an actor to play each character, and so on and so forth until you have them clear in your mind.

Then bring them together however you decide to do it, and let them go for it.

Characters and a situation--Stephen King says that's all you really need. Try it and see if he's right.

I may have to take the advice of S. King and try not to 'pre-plot' my story more than needs be and try to establish a working story.

In fact, it may be easier to show deeper into a characters depths in sf and fantasy because you can use utilize so many different situations than you can in your typical literary work.

Just read OSC, and you'll see this demonstrated. Card has written characters that feel more "real" than those found in many "serious" works.

It is the author and the writing, not the genre that determines how deeply you see into a character.