Describing characters at once or throughout?

I think it's absolutely ok not to describe your characters at all. The worst kind of character descriptions are descriptive paragraphs that are clearly describing characters. As a reader, that's where I stop caring. That said, some people need visual cues and like to 'see' who they're reading about. Feel myself start tapping impatiently on my book cover if there's more than a few lines about somebody's appearance unless it's making a comparison or a deeper point. Try describing a character through their dialogue, thoughts and actions when they are doing something else. Saves time and reinforces your sentence. Hope that helps. And good luck with your novel.

Rein yourself in if you have hair, eye colour and build in the same line.  Others will disagree with me. Read any piece of work by ABC member Celticman and study his characterisation. That's the way to do it. 

'Her eyes they shone like diamonds

Her neck it was white as a swan

And her hair it hung down to her shoulders

Tied up with a black velvet band.' (trad. Irish song Black Velvet Band sung by the Dubliners)

Sometimes description matters! Without reading more of your story I have no wish to give my amateur advice.

Your second line 'He swung his arm in anger...' is very effective.
 

I think the important thing is only describe as much as you need to. In most cases the reader can supply their own imagery - and should be encouraged to do so. For example, Elsie's example from the Dubliners 'Black Velvet Band'  does this by leaving much for the audience to fill in for themselves. How tall? Skin tone and colour? Build? That's how you engage your reader - by letting them finish the puzzle for themselves. And remember, even the most minor of characters should be sketched in at least enough to give them a reality.

Like Vera, I like to leave it vague. That way the reader can build up their own image. I add in the odd bit for clarification, and only when needed. Celticman is indeed good at this, I'll second Vera's sugestion to read him. Vera is also very good at this. Also Scratch.

Good luck with your novel!

Depends on the nature of your story and weather or not his/her/its details propel the story forward or prevents the plot from moving forward. By not ascertaining a characters background this provides an air of mystery and curiosity around them which gives the readers a sense of direction as to why they are doing what needs to be done or leaving the readers on a razors edge of tension since a character without a history makes them utterly unpredictable. If you do give details of your character it must only.be used for irony or some sort of action-reaction that plays out later in your story; like the classic revenge story or man against nature. Everything about your character not only defines the nature of your story it also effects the readerq ambition to continue on with the plot. Choose wisely mes brave.

It depends on the story and if it's needed.

Big chunks of description can be a bit obvious and contrived. See how it flows, do what comes naturally then either rerad t back, out loud, or get someone else to read it. I personally find too much detail annoying, little charecteristics are good, things that make them stand out, but their height (unless it's a prominent part of the story) etc are not needed. I have an imagination and am capable of picturing somebody without you putting every little detail in there.

I agree with VeraClark that lengthy descriptions are off-putting--and quite frankly uncomfortable to read at times. When the writer goes into too many details and you're given the exact measurement of the spacing between a character's eyebrows and the likes... I feel weird just reading about it.

I've recently started writing a sci-fi novel, which is my first ever attempt at writing a story with characters and dialogue. I'm approaching the end of the first chapter now, and I still haven't described any of the characters. I figured I'd add some more details as the story went along, as/if needed. I think it's more important to establish a character's personality. I care more about who they are than how they look. I've been working on this project for 20 years, and I still haven't even pictured the main characters in my head.

Maybe it'll help to think in terms of your character arcs - who they are at the beginning, who they are as they go along and who they are at the end (in other words, how they change) - that'll give you a clue as to the degree of information you can/need to reveal about them as you go along.