There's a world around us, full of flowers and kittens and people and bandits and guns and nukes and... But there's a world first of all. I think that you've already know that I don't believe in a vacuum. You probably know that for me, the now matters the most. The now is the thing that matters, and the world is the thing that we fight for and strive for.
The world is there waiting for us to make it bigger, richer, fuller, all in the goal of making it come alive. If we're supposed to fight for the world, the world should well deserve it, isn't so? Making the world come alive can manifest in many ways. It can manifest with our descriptions and with our mood and atmosphere and tone and... But it can also come alive from the single most overlooked element: Normal life of the world's habitats. If Ms. Lucy and Mr. Robert are there, doing what a normal newly-wed couple are doing, if on the other side of the city the lord Radolph is there giving a speech and if in the dark corners of the alley a little girl is stealing money to save her dying little brother from his death, the world will be much fuller and will naturally come alive.
Look for example on the movie Paranormal Activity. It starts really slow, with the presentation of our 2 protagonists, and they're not fighting monsters when we first see them. They have, and that's important, a normal couple conversation. They fight about what a normal couple will fight about and talk to one another the way a normal couple will talk to one another. This is the whole thing we see in the beginning, and this is the most important beat in the story at this stage and many stages afterwards. You can go bump in the night if your characters aren't real (you can, actually, but it will be very very shallow and uninteresting...).
Truth is, these are the little details that matter the most; these are the things that deserve fighting for; most of all, this are the things that make an imaginary world come alive. Too bad they are mostly overlooked, don't let them be forgotten again, they need you as much as you need them...