I have been doing some work on character development for my novel. I found that though I can write pages about the main character, other characters, who have an important role and will be there for most of the story(ie, they are not minor characters), only get a small paragraph.
So my question is: Other than the hero, should I do some work trying to develop these other characters as well? At the moment, they are just 2D figures, with little personality.
Complex, well-rounded characters can add a lot to a story; on the other hand, lots of good stories manage without them.
What you want to avoid is characters that feel flat, unbelievable and/or cliche. That's because if the reader feels that way about a character, he feels that the story is false, artificially constructed. The reader will often react in one or both of the following ways:
If your writing isn't outright bad (which, hey, it might be), then simple characters might work quite well for a lot of types of books. The rule of thumb is, the more central character, personality, and relationships are to the book - the more crucial it is for the characters to be complex and detailed. In other words, your character work needs to be strong enough to bear the weight you're going to put on it - you can't let it be less; it doesn't have to be more.
For example, you can't have a deep exploration of your protagonist's soul and his relationship with his dysfunctional family, if you portray him as being deep and complex, and the family as a bunch of twits. It doesn't match up - and calling the reader's attention to the depth of your protagonist's character will have them looking for the same in the rest of the cast. It'd be like watching a fight between Dostoyevsky and Bugs Bunny.
But even when you can manage without them, having more complex characters is a major plus. It contributes to a sense of depth and realism; it avoids not only bald cliches but also the familiar and not-terribly-interesting; it earns more reader involvement and investment. Of course, this too can be taken too far - you don't want to swamp your story with irrelevant details, or lose reader interest in the protagonist because you're trying to devote attention to a bunch of characters who really aren't very central.
The concerns I've laid out here are, IMHO, good ones to guide you when developing and portraying characters; from here, what you need to do is be aware of these different concerns and considerations, and find a good balance them that works well for your characters and story.
The short answer is yes.
The long one is definitely.
the tl,dr of below is: If you don't have a great cast of supporting characters then the work you do on your main character runs a severe risk of being undermined, and hence wasted.
To expand into the territory of "why". Presumably your main character hits all the buttons for three-dimensional i.e.
And you might even have gone into:
And so on and so forth.
In addition your dramatic action will be conflicted. If your action is allegorical then your protagonist, with all those dimensions and that, may visit dimension four because real people in allegorical situations can find things... difficult, to say the least.
If your action is not allegorical then again the action will mirror the protagonist in that it will not be black and white (like allegory is) but all different shades of grey.
So there's every possibility that some features or actions of your multi-dimensional protagonist are not what might be seen as sympathetic. Someone might just turn around and think your protagonist was a big, fat jerk for reacting to the story incidents the way they do.
As the author you have two important weapons in your arsenal against this reaction, your antagonists and your supporting protagonists.
In the case of the former it is in your best interests to make those who oppose the protagonist as three dimensional as possible because a character can be judged by the strength of their enemies. If the enemies are cartoonish or not as rounded as possible, if there is nothing magnificent about them, then it reflects poorly upon the character of your protagonist. If the antagonist is, in many ways, charismatic and wonderful, but just wrong, it shows strength of character in the protagonist to respectfully stand against them. Even if a little more moustache twirling is involved the dark charisma of the villain makes the hero look even more glowing by comparison.
In the case of supporting protagonists this is even more so. The best gal, the sidekick, the childhood friend, the staunch colleague etc. all are really reflections on the kind of people who the protagonist associates with. All of them are people the protagonist cares about and all of them care about the protagonist. In either case the idea of being held in high regard by people with the personality of flat-pack furniture is about as unappealing as the thought of a hero who cares deeply about someone who has not even the personality of a cheeseplant.
In addition the villains may not always disagree with the hero and the supporting heroes may not always agree with the main character.
When you write scenes to demonstrate why villain A and the protagonist are eerily close in one regard or another it necessarily sounds a note of warning. The hero could go bad. Drama! Horror!
When you write scenes of disagreement between protagonists it serves to reinforce the previous case. But it also shows that relationships are hard and if a hero is to succeed then it will be with the assistance of those closest to them who respect them enough to fight for what's right even when their friend is being bull-headed. A note of drama, sympathy, a possible rift should be a thing too terrible to contemplate for the reader. Will friends become enemies? Will enemies become friends?
Such stuff is the lifeblood of dramatic tension.
Finally, your protagonist, unless you are some kind of intellectual masochist, probably embodies some ideals you find admirable or, indeed, heroic. The presence of weak antagonists and nodding yes-men in support protagonist roles shows a weakness to your belief in those ideals. To make the protagonist a paragon of unassailable virtue, the antagonist a thoroughly despicable coward and weakling and the support roles a bunch of simpering nodding dogs shows an unwillingness to counter your own ideals with anything substantial.
Fully developed characters, united not by ego but by a dedication to a principle that, even with flaws, is held to be correct, will aid in the construction of compelling fiction. At the end of the day real life is not even that simple, who can say, in reality, that we ever really do something wholly good or wholly bad? We just make the best of what we have at the moment. The closer your own fiction comes to giving the impression that this is what's happening in the story, whilst simutaneously presenting the gentle and entirely apprpriate lie that things can be much simpler and easier to believe in, the better for you and your readers.
EDIT: As I say in my comment the rule of thumb, rather, should be: You can have a whole host of entertaining two dimensional characters and that's cool, or a complex and brooding network of three-dimensional characters and that's cool too. But, to borrow a metaphor from the ghostbusters, one should not cross the streams. So you either ditch the character development and simplify your main character or you beef up all the other characters. Mixing both can just seem jarring.
The answer already lies in your question: "they are not minor characters ... [at] the moment they are just 2D figures, with little personality."
So, yes, you need to give them a lot more development. You want them to be 3D characters, but at the moment they're 2D characters.
Whether the work put in for these other non-minor characters will be equal to the amount of development you put into your main character is hard to judge without knowing more about your story, but you should reach a point where you don't have any doubts or questions about who these people are, how they relate to one another, and their role in your story.
I agree with Craig Sefton that your characters need to be more than flat, but I'm a little worried about your "only get a small paragraph" line.
It's important that your characters be rich and vibrant, but it's also important that we learn about them through their actions, NOT through you telling us about them. If your 'small paragraph' is in your notes, that's fine, but if it's in your actual story, I would say that a small paragraph is actually more than you need, in terms of introducing a character. Most of the characterization should be spread throughout your story, not dumped on us all at once.
If that's not what you meant by 'small paragraph', then I'm with Craig. Flat characters are fine if they're incidental, but if your MC interacts with them, your story will be much more interesting if you give them more depth.
I would have to say that the extent to which you develop your characters determines how "main" a character actually is. Even if a character with less exposition is more important to the plot, a character with more exposition is likely to be easier to identify with, and thus viewed as a main character.
