I hate it when heroes in movies get lucky; when they find a trap door, a convenient air duct, a waiting motorcycle, or a ladder at the precise moment when such things are desperately needed. Not that chance and coincidence don't play an integral part in movie land, but real life doesn't place the solution to your dilemmas right into your hand - it suddenly points to a solution that was there all along.
Early in THE MATRIX, the hero stands in an office, getting chewed out by his superior. Outside the high-rise window he sees window washers working on a scaffold, cleaning the windows. Later in the sequence, the hero is ordered to leap onto the now-empty scaffold to escape from the bad guys.
Now, the directors could have just had an empty scaffold sitting around. The building is a major high-rise, after all. A convenient scaffold wouldn't have raised too many eyebrows. But they placed the scaffold earlier on in the sequence so that the image resonated, so that it seemed a preset part of the setting instead of a surreal coincidence. At the crucial moment when the hero looks out the window for an escape, instead of A scaffold, he sees THE scaffold.
So the hero turns away from his bitchy superior and sees the workers on the scaffold. But if he had merely noticed the scaffold and turned away, we might have known that perhaps the scaffold would be required for a daring escape. It could've been a giveaway. So the directors gave the scaffold a dual purpose. The window washers squeegee water away from the window in a symbolic shot, hinting at a false reality that is about to be pulled away from the hero's eyes.
THE MATRIX is not a perfect movie (despite how much I babble about it), but the above sequence is a great example of placing an object or person in a scene ahead of cue, even if its usage later on is relatively incidental. Of course, you don't have to do this in your screenplay. You can have your hero find extremely helpful objects in his pocket, or have his sidekick suddenly turn out to be fluent in Swahili if that's the language the treasure map is written in, but going the extra distance to weave key objects, etc. seamlessly into your script is not just smart, but classy.
It's not just classy, it's artful. Creating a world is about more than just erecting walls and tacking down carpet; it's about the crooked painting hanging on the wall. It's about the details that stand all around your characters and the part that those objects play in your characters' fates. If you draw attention to "a crooked painting on the wall" in your hero's apartment, is that just a clich?d example of your hero's slobby, disjointed life, or is it the painting that hides the safe? Or both? Ok, that's a terrible example. But hopefully you see what I'm talking about.
Chekov's famous formula goes something like, "If a gun is on the mantle in the first act, it must go off in the third". To this I'd add, "and the gun should have a reason for being on the mantle in the first place". Not just for Ibsen-esque symbolism or a glibly explained reason, but an authentic reason - something that makes sense, so that we don't sit through the whole screenplay or movie waiting for the trigger to be pulled.