This seems appropriate to post on Moore’s 60th Birthday.
Alan Moore had some interesting thoughts on the subject in an essay he wrote in 1985, later published by Avatar Press as Writing for Comics.
The approach to characterization in comics has evolved, like everything else in this retarded bastard medium, at a painfully slow pace over the last 30 or 40 years. The earliest approach found in comics was that of simple one-dimensional characterization, usually consisting of “This person is good” or “this person is bad.” For the comics of the time and the relatively simple world that they were trying to entertain, this was perfectly adequate. By the early 1960s, however, times have changed and a new approach to characterization was needed. Thus, Stan Lee invented two-dimensional characterization: “This person is good, but has bad luck with girlfriends,”and “This person is bad, but might just reform and join the Avengers if enough readers write in asking for it.” Again, at the time, this was breathtakingly innovative and seemed a perfectly good way of producing comics that had relevance to the times in which they were being produced. Progress since that point has been minimal. In an effort to keep up with the times, the characters themselves have become more extreme, brutal, bizarre or neurotic, but the basic way of portraying them as changed very little. They are still carefully defined two-dimensional characters, maybe with a little verbal window dressing thrown in to liven them up.
I haven’t read the essay in sometime. I assume the “retarded bastard” is a reference to the origins of the medium, and less harsh in the longer context.
He talked about his objection to 15-word characters, who can be summed up quickly. An example he gives is “a cynical police officer whose parents’ murder leads him to wage masked war against crime.”
If the writer is clever, minor quirks of personality might be introduced into the scheme.
If the writer is adventurous, he might feel the need to explore the character in greater depth The problem is that however deep the pool of the character’s soul may be it’s still only 15 words wide.
The point is that since the initial working assumptions upon which the characters are built are limited and increasingly unworkable, so too are the characters themselves. If comic writers are going to solve the problem of developing their level of characterization to a level where it’s in keeping with the times, perhaps it wouldn’t be a bad idea to throw away some of these outmoded templates and come from at the problem from another angle. A logical place to start would be to simply go and look at real people. Consider the character makeup of people around you and consider your own personality as well, in as cold and objective a light as is possible After a little while you may discover that almost nobody can be summed up in 15 words, at least not in any meaningful and relevant way. You might also notice that people change their personalities depending on who they are talking to. They might have a different voice in their conversations with their parents from the voice they use when addressing their workmates. They vary their attitude and their mood hour by hour. Often they will do things that seem completely out of character. Simple observations such as these help to gear the creative mind toward a more complete understanding of characterization than can be afforded by any snappy little generalization about the phenomenon.
Two-dimensional characters have the capacity to surprise, in a way that still fits the character (to distinguish from a one-dimensional character who does something unexpected because it serves the plot). This is pretty much the norm for recurring characters, but it could still apply to a few one-offs (Tim Hammond comes to mind.) There’s a bit more complexity. And some contradictions, along with explorations of motives.
Three-dimensional characters are the most complex, nuanced and fully realized. And they’re pretty rare, as it requires a greater amount of work, much of which could be lost on the reader. The greatest distinction between 2-D and 3-D characters is the level of understanding about how they’ll act in different settings, and how they may change over the course of a story. But it’s also the degree to which you might come to understand things about character on your second and twelfth read of a story.
At some point, I determined that this was relevant for the Spider-Man characters.
Ideally, Spider-Man should be three-dimensional. Anyone with their own monthly title should be three-dimensional (or close to it) when they appear in Amazing Spider-Man. This includes Flash Thompson and any guest-starrs. They have to be able to keep up with Spidey. If Peter’s in a serious relationship, it’s preferable for his significant other to be three-dimensional, so that she can also keep up with Spidey. An antagonist could be three-dimensional. It’s not always a requirement. And it doesn’t always have to be a supervillain. It could be a rival at work. A supporting character you want readers to be emotionally invested in could also be three-dimensional. But they need an arc of their own. Even if it’s going on at the same time as Spidey’s story.
I’ll go with two other arguments, paraphrased this time, as it’s time-consuming to go with the exact quotes.
In Story, Robert McKee argued that it’s problematic when minor characters are developed too much. Audiences are conditioned to believe that someone who is especially interesting will reappear, so a one-off character (he uses the example of a taxi driver in a two minute scene) could be too well-developed if the audience expects some sort of reunion. Audience loyalty could be divided, if a character is more compelling than the protagonist. He suggests that Blade Runner might have been a flop because Roy Batty was so much more interesting than Rick Deckard.
On the other end, Heath Ledger’s Joker stole the show in Dark Knight. And that was a monster hit. I just watched Giant, in which James Dean took a supporting performance, and made it into the most compelling character in the film. That was a monster hit in its day.
In a Writers Tale, Russel T Davies, former Doctor Who showrunner, expressed his belief that characters are most interesting when they act contrary to how they normally act. So this would be when a character who is usually selfish makes a great sacrifice, or when a character who is always calm shows some rage. This could be one way for “flat” characters to become “round.”
What Would Spidey Do? 
Characterization is one of the hardest parts of being a writer. As an editor I have had to ask writers many times, to clarify or expand characters because I could not keep the characters straight. As a writer I feel that I sometimes fail at developing minor characters too much and have been called on it by some of my readers.