A Mainstream fiction story as compared to a SpecFic. Is there a real difference in methodology other than the setting?
For some reason I've had the impression that a mainstream work is essentially a snapshot of a life situation.
Some thoughts on that...
Remember that genre is really a marketing artefact. That being said, to my view the biggest difference between mainstream and specfic is the contrast between First World and Second World assumptions.
A mainstream story doesn't have to define anything outside the character and conflict embedded in the story. Which is not to say mainstream stories don't range far more widely, just that their genre doesn't inherently bear that requirement. These stories live in the world that the reader already understands, or at least a version of it.
A genre story has to define everything, at least by implication. (Not on the page, though!) These stories live in a world where the reader is consciously expecting novelty, and generally is watching for the appearance of that novelty.
To go back to a very simplified example I've used before, when John Updike says "Rabbit is rich," any modern American reader knows that he means someone named Rabbit is wealthy, either in money or in some metaphorical manner.
When a genre writer says "Rabbit is rich," "Rabbit" could be a person, an animal, a spaceship, a civilization, food, or something else entirely; "rich" could be wealth, a state of humorousness, fuel/air mixture, fattiness of the sauce, and so on.
Mainstream fiction does often use genre tropes — The Time Traveler's Wife and The Lovely Bones spring immediatey to mind as recent, highly successful mainstream works which are undeniable embedded in the fabric of genre. But when mainstream uses those genre tropes, their are introduced as exceptions to the assumed norm within the narrative. They're not present to satisfy the novelty-seeking expectations of a genre reader, they're present to provide a lateral illumination to the dramat expectations of a mainstream reader. This is how Magic Realism works, for example -- introducing the miraculous into a set of First World expectations and experiences.
Genre fiction uses its own tropes as the default. An important aspect of the story experience for the reader is how explicitly those tropes are revealed or explored. It doesn't make sense to speak of introducing mainstream tropes to genre, because the First World is already embedded in the Second World. The Second World exists as a response to the First World, after all.
This argument becomes slippery when you have genre set in the First World — hence the blurry boundary between urban fantasy and paranormal romance. "Urban fantasy" is a genre label which implies Second World story characteristics through the use of the term "fantasy", while "paranormal romance" labels a First World story with unusual exceptions. Different set of reader expectations.