In a previous post I mentioned the quote: "Every pixel you take out of the imagination and put on the screen is a pixel you are taking responsibility for."
I swapped a few words in that sentence and got something even more relevant to where I stand: "Every word you take out of the imagination and put on the page is a word you are taking responsibility for." This goes way beyond "show, don't tell" (which I think should be "show and tell in the right measure").
By "the imagination", I'm referring to the imagination of the reader, not just the writer. Every time you call attention to something, describe something, you are asking the reader to surrender that much more of their imagination for the sake of yours.
This is not a bad thing; sometimes you have no choice but to do this. The reader has no way of knowing what your protagonist looks like until you say something on the matter. The trick is to know when to stop.
It's rarely hard to know when to start. A coherent story has to begin somewhere, with descriptions that give the reader someplace to put their first steps when they land on a page. But it also has to end somewhere — not just in terms of the storytelling coming to a halt, but the author stepping back and letting his words do their work in the reader's mind.
Every writer seems to have a different process, a different set of thresholds, for sussing out where to begin and end with this. Georges Simenon, master of brevity, would never have spent the amount of time on a description or a scene setting that Dostoevsky did. But neither of them were working to the same ends, and neither of them should be seen as being the "better" author because of it. Granted, I've done my fair share of grousing about the lack of brevity being a drain on good spec-fic writing (Wheel of Time, I blame you), but I'm backing off from the position that brevity is a requirement.
In SF&F, it is tempting to succumb to what in Japanese popular culture has been called "too much explanation disease": the urge to explain and explain until all possible pleasure of discovery or insight on the part of the reader has been bled out. This is more than just the misdemeanor of stopping the action cold to dump information in the reader's lap by one means or another; this is also the felony — often all the more dismaying for being done in earnest — of explaining a character's behavior to us, sometimes at the additional risk of having the explanation we receive not jibe one bit with the actual character we see portrayed on the page. An author who stamps his foot about the heroism of a given character, only to show us a selfish git with an ego the size of the moon, is not doing himself or his audience any favors.
It's been said that the single most difficult thing in fiction of any kind is the successful delineation of character, which may explain why it's often easiest in many kinds of stories to fall back on tapping into the cultural shorthand that the author knows the reader also has at hand. It's easier and faster to stick The Hooker With A Heart Of Gold into a story than it is to show us, say, someone doing something she hates for what she once thought were good reasons only to find herself beaten down even further by it than she ever dreamed. It's difficult to show such things because few of us have access to something like that in our own lives, or would want it — and if we did, we might not know how to make it into part of our story without seeming grotesque or maudlin or both.
The same problem becomes all the more complicated in SF&F, when you are faced with the prospect of inventing people who are products of social circumstances that simply don't exist in our world. Or at least, that's how it might seem from the outside. As long as what you are writing about is recognizably human, or has enough tenuous connections to the human to be described as such, then you owe it to yourself to mine what's human around you for their behaviors. When Ridley Scott and Dan O'Bannon developed a story about a spaceship crew meeting a menace, they left behind the Intrepid Explorers template and instead used one a little more down-home and accessible: Truckers In Space. Thus came Alien, which worked so well in big part because its crew were not bold and brave confronters of the unknown but underpaid, harried, blue-collar greasemonkeys. From what I've seen of Prometheus, though, Scott seems to have gone back to the Intrepid Explorers template.
So how do you show such people off without talking them to death, especially when you're dealing with a world which may not be ours? For starters: give those people freedom of speech and movement. That requires having a character who has some roots in real life as you have experienced it, because only then does it become possible for them to do the talking. If your story pushes them into a corner (and all great stories are about people pushed into a corner), let them push back as they would. This tactic is not only useful for the delineation of character, it helps weed out characters from your story too: if they push back in a way that just isn't interesting or is downright counterproductive, then perhaps you've picked the wrong person to talk about.
Once again I must fall back on Milton Glaser for the last words. When faced with the dictum "Less is more", he struggled with it. Sometimes less is just less, and sometimes more is just more. Perhaps better to say "Just enough is more". I have, in my own way, come to the same conclusion. Your people, no matter what their origin or their circumstances, need to be given the freedom to stand on their own feet and speak for themselves. Omnipotent author you might be, you still cannot do this for them.