We hear a lot about how it's important to get to the core conflict as quickly as possible, and not get bogged down in backstory, to keep pacing up in our stories. Indeed, quick pacing is one of the characteristics of YA literature that has made it so successful, and I certainly have seen a trend toward faster pacing in adult literature as well. On the other hand, if you leap into the core conflict of a book, or even of a scene, too quickly, you'll be doing yourself a disservice.
The impact of any one sentence, paragraph or scene does not stand on its own. Think about horror. There is far less shock value in coming upon an appalling scene if your character has not had a chance to build up fearful anticipation, and your reader has not had a chance to feel the increasingly spooky ambiance of a place. For fantasy and science fiction - and indeed for mainstream work - the principle is the same. Each piece of a story stands upon the piece that came before it. It's therefore critical to give some thought to what kind of foundation each piece of your story needs in order to be maximally successful.
In my current novel, I have an instinctive sense of the pattern that needs to be followed for each chapter. The chapter needs to begin with tension, a goal for my protagonist and something that makes that goal risky or difficult to accomplish. Because of the society in my book is very tightly wound and people's success depends greatly upon reputation, very often setting up the initial stakes means setting my protagonist's goal up against a background of public watchers who may take his actions amiss, and pass that on to others of influence. Then as I go further in, the main conflict develops and is influenced by that background, much in the same way that the flow of water is influenced by the presence of rocks or islands. At a certain point in the scene comes what I have been fondly calling "the left turn" - a change in the situation that abruptly raises the stakes and gives an entirely new meaning to everything that happened before.
Here's an example. My protagonist, Tagret, gets invited to a tea party - not just any tea party, but a party given by his father's worst political enemy. The risk here is higher because previously, I have set up the fact that the woman who invites him has seen him with a young lady his father wouldn't approve of. Thus, Tagret goes to the event feeling that he has been blackmailed into attending (raising tension). He decides to bring along his best friend so it's less clear to any people at the party whether he or his friend initiated the decision to attend. This decision is logical given the social stakes, and Tagret feels that having his friend along will make it easier for him to get through the event with his reputation intact. At the tea, his father's enemy makes a compelling case for why Tagret should be on her side rather than on his father's (which he is only reluctantly anyway). The strategy of bringing his friend seems to be a good one even though his father's enemy manages to speak to him alone. But then an external emergency crops up, and suddenly all the people in the room - but most particularly Tagret and his friend - are in danger because they are not at home where they can be safe. Tagret's mother's servant appears to take both of them home to safety, but now Tagret's leaving the house appears irresponsible for an entirely different set of reasons, and bringing his friend along has become an even more irresponsible act because his friend was also exposed to danger. Even the servant who takes them home must suffer because she is seen as having been complicit in Tagret's disobedience and risky behavior.
All of the pieces of this sequence link together. The enemy seeing him with the girl, the invitation, the decision to take his friend along, etc. I almost began this scene at the point when Tagret was just arriving at the tea party, but when I did that, the tension and stakes didn't seem high enough. I hadn't thought through the lead-in thoroughly enough to realize that Tagret would take his friend with him to try to protect himself. If I had in fact left Tagret's friend out of the scene, I would have lost much of the larger impact that was brought about by the change in stakes. The change in stakes, i.e. the larger emergency, has huge consequences and puts Tagret's life in danger - but it hits home much more effectively if his friend is with him, because Tagret endangering his friend has a far more personally damaging set of consequences. What seems like a cautious and safe decision in response to one set of circumstances is in fact what lets the later set of circumstances be far more dramatic.
While somewhat different, I think this question of foundation and setup is related to the question of how and when to include character backstory. Very often I hear people talk about backstory in terms of "information readers need to know to understand what is going on." I'm not sure that's the best way of thinking about it, however. I like to think of it in terms of the character I'm working with. The backstory this character has is the foundation for his/her understanding and emotional reactions to events. Similarly to what I spoke about above, where scene setup helps give a foundation for the emotional impact of the core conflict and the raising of stakes, the inclusion of backstory detail gives us a foundation for the emotional impact of events on our character. This then translates into the impact of those events on the reader. Think through backstory just as you would think through scene setup, identifying those critical elements which are needed to support a given character reaction. Then, include those elements. Usually what this means for me is a clearer way to identify which elements are necessary and which are not. The smaller elements that support a character reaction are much easier to build into the narrative subtly, without having to resort to paragraphs (or pages!) of backstory explanation.
It's something to think about.