Depends on how do you approach role playing games.
Do you know about the gamist/simulationist/narrativist theory?
Basically, there are 3 main reasons why people want to play RPGs.
Some approach it as a game, they are players out for in-game rewards. Send them of to find a long lost treasure (guarded by a powerful monster or any variation thereoff), no matter of filmsy the justification, they will be happy. However, you gotta make sure that the reward interest them to get them interested in your campaign. Those are the gamists. The campaign (adventure hook - reward ) is the most important thing for gamist.
Some approach it as a way to mimmick real life, but in a fantasy world. They will want to see a lot of fluff and realistic npc interactions. These guys want to feel they really are part of this world bigger than themselves, but you gotta make sure that they actually enjoy the world you set out for them to adventure in. These guys love freedom more than any other player archetype, so even if you have to plan the world in advance, you have to be able to improvise a lot and be flexible about what adventure hook they will take and which ones they won't. Those are the simulationnists. The world building is the most important thing for simulationnists.
A last but not least group approach rpgs as a story-telling medium. They will have fully fleshed out and complex characters with intricate backstory. They did not include all this backstory just for the fun of it thought, they expect you to actually use it. That guy is married and loves his wife? Well, the wife's better be an important character of the campaign or your player will be very disappointed. Maybe this guy plays a character out for revenge for the murder of his brother. Well, you gotta include revenge as a major theme of the story. These guys care less about realism than they care about drama. That underdog would never, in the real world, defeat the goliath? Who cares? If it's dramatic, include it in. Perhaps you will want to tweek fighting rules a bit to give characters more opportunity to rescue their comrades. (instead of separating the fighting into rounds, separate it into limelights, with a character's turn lasting until something interesting happens. Don't keep track of the time flow of combat, the whole point is that when a character gets in a tight spot, the other characters will always be able to attempt a rescue). Actual role play and a genuinely interesting plot is of paramount importance for these players.
The most important thing is to agree with your players on what you want from your rpg sessions. Enjoy a fun gaming night? build a fantasy world? tell a story? Those things require a very different kind of work on your part.
Talk to your player, let them on the inside, sometimes, you can even let them come up with their own ideas for the campaign/world building/story/characters. You could even let them roleplay some NPCs from time to time, especially if they are relevant to their character. ( I, for one, always let my players roleplay their character's relatives and their personnal ennemies : the player probably knows these characters more than the GM =P ).
