I got the idea for this post from a thread on the Absolute Write forum, in which a member was asking about how to come up with fresh ways to describe characters' appearance.
It's interesting, but the question of fresh ways to describe appearance does contain a fundamental assumption that character appearance must be described. This isn't always the case. There are plenty of times when readers are happy to fill in an appearance from their own heads, and don't want to be bothered with the details - this is especially true when the action of an exciting story is flying along. So I thought I'd break the question into three parts: who? when? how?
You don't want to describe everyone. When we walk around in the world, we take in everyone's appearance on a basic level, but we don't have to remember it - and our brain takes it in easily as part of its massive parallel processing. When you're writing a story, you have to take that massive stream of parallel input and render it in a single stream of words. The result is this:
The more of the parallel stream of experience you try to capture, the longer the single stream of story language gets, and the more the perception of the passage of story time slows down.
So my general rule for describing appearance is as follows: describe only those people who deserve to be noticed and considered for a non-negligible period of time. I put the rule in this general form because I know some folks out there are writing with character-external narrators. Your narrator is going to have an enormous influence on how you should treat the problem of description (whether it be of characters or setting). Different narrators will concentrate on different things. With an omniscient narrator, the biggest question is who and what in the story the reader needs to notice. With an in-the-action character narrator, the biggest question is who and what in the story that character notices. With an out-of-the-action character narrator, such as a retrospective narrator, you can include stuff that the character noticed in the action, and stuff they noticed or recalled, or looked up, after the action finished. I talked about noticing in my earlier post about unreliable narrators (which some may find illuminating for this discussion, so it's here.)
I should add that I mean these suggestions to apply to all characters - including the point of view character. Ask yourself, for the sake of argument: How much time do I spend focusing on and maintaining my appearance? How much does the question of my appearance intrude on my daily judgments (of, for example, how other people treat me)? If your point of view character doesn't care about his/her appearance at all, and doesn't feel it's relevant to the way others treat him/her, then really there's no reason to slow the reader down and get into details - not to mention that there's no natural context for it.
Different authors will likely treat this differently. When I'm powering through and drafting a story, I usually don't even think about the who/when/how questions on a conscious level - I just write. However, when I go back and edit, especially when I've been told by a critiquer that I need to include a description of someone, I try to look for what I call natural context for description.
Natural context for description is that moment of noticing and consideration. When does it happen, and for how long? When you pass someone on the street, what do you notice about them? Chances are it's not going to be their height and weight, unless you've been trained to give police evidence. In a split-second glance, you might notice just one feature - unusual hair, or clothing. If you're close enough to them to be potentially in their sphere of influence, you might notice their size relative to yours. If they are nondescript in a crowd, you might not notice them at all; if they are nondescript but alone, probably all you'll notice is their presence or absence, and maybe a bit of the surrounding context of where or when they appeared. If they are totally and utterly alien but moving too fast to be stared at? You might only notice they were alien. And maybe the color of their blur as it moved by.
Naturally, then, the longer the period of time you have to look - and the better reasons you have for doing it - the more you're going to notice. A character may notice one feature of a person on first glance (in which case, mention it at that point in the story), a few more features in a second encounter, but not actually be able to put together a detailed impression of a person until they've met face-to-face for a period of time.
I've heard people remark that romance includes a lot of description. Well, it does. But there are two very natural reasons for this. First, when we're romantically interested in someone, we're going to spend a lot of time and resources considering them on all possible levels, the easiest of which is appearance. Second, when we're romantically interested in someone, our senses are heightened, which enhances our ability to notice small things in our surroundings. Bang, there you have it - more natural context for description, even independently of authorial style and choices.
Contrast this with moments of high action in a story. Action is where you're going to want to keep description to a minimum. When things happen fast, we don't have time to notice a lot. And when we want readers to perceive that things are happening fast, we don't want to add the extra words that will cause them to feel story time slowing down.
Wow, we've finally gotten to how. Let me start by saying that some of the aspects of "how" have already been touched on in the question of "when," at least where it concerns how much description you want to put at different points. So I'm going to try not to concentrate on that, and take the discussion in a bit of a different direction: the noticing and judgment direction.
What one person notices about another at any given time will depend on a lot of personal and cultural factors. Even a split second judgment of "what stands out" isn't necessarily going to direct everyone's attention to the same things. Someone with police training will be able to form split-second descriptions of height, weight, and distinguishing features. Someone with spy training will have very different impressions - possibly notice whether the person is armed, or moves as if they have martial training etc. Does their species stand out most? If they're alien, or if the observer is an alien seeing a human, then maybe so. Or if the character is an older woman judging a younger one in a social context, she may bring a large set of expectations for proper behavior and appearance, and run down a basic checklist of these in her initial judgment of the younger person.
To go back to the question of freshness and originality - if you write your description as befits the individual judgment of a unique point-of-view character, including that character's species, cultural background, individual experience, and social role in the context of the encounter, you're far less likely to describe people the same way twice. And the more time the describing character has to pass judgments, the more divergent these things can become.
Just for fun, here are a couple of character descriptions from my alien points of view.
Allayo the Gariniki describes humans:
"My captors have the faces and ruffs of simians, but are too large and have no tails at all."
Rulii the Aurrel describes human hands:
"Parker shows me his hands: short, stiff, single thumbs placed far to the side; long, long, soft brown fingers ending in covers like pale coins instead of claws. Hand-showing: a human submission move."
Part of why I include these examples is to show difference; part is to show that one character's description of another does more than describe the other: it also reveals things about the one doing the describing. We describe things and people in terms of what we know well. So from Allayo's description we can learn that she knows about small monkeylike creatures with tails. We can also conclude that she isn't one of them. Though we don't have enough information to construct her full appearance, we can be put on the alert to look for more cues to her actual appearance as we go forward (she's a reptile). From Rulii's description we can conclude that Aurrel hands have more than one thumb, that his thumbs are long and flexible, and his fingers are short and have claws. We also get a hint that he cares about submission behaviors.
Comparison between two characters is a great tool to provide natural context for description, especially for description of a point of view character who isn't naturally vain or otherwise focused on his/her appearance.
One last thought: while many times it's great to leave a character's precise appearance to the imagination of the reader, it's worth establishing that character's physical characteristics if they will be relevant later in the story. You don't really want someone getting halfway through your novel before they realize that they thought the main character was a brunette and the author thought she was a redhead. That level of strategic placement of description is unique to the author's point of view, and can often lead you to have to stick descriptions in where the character didn't naturally fall into them on the first draft. But if you keep your eyes out, you can usually find an opportunity here or there to provide a few extra words in a natural context. And then it will all work out.