It is a common misconception that round characters are a good thing in novels and that flat characters (also known as "cardboard" characters) are bad. The truth is that you need both types in a novel.
But make your major characters, particularly your protagonist, too flat, and your minor characters too round, and you will have a problem.
If flat characters are stereotypes defined by just a single trait - a short-tempered businessman, for example - round ones are impossible to label in this way.
For every characteristic they have which places them into one pigeonhole, they have another one which works against it.
So if one of the businessman's traits is his bad temper, you could counterbalance it by giving him - I don't know - a love of ballet.
Whenever we meet anybody in real life, we are all guilty of "categorizing" them, of thinking we know everything about them based on our initial stereotypical impression of them.
When we actually spend some time with that person and get to know them, our initial impression (even if it was accurate) will be altered by traits which work against our stereotypical view of them.
And it is exactly the same with a three-dimensional character in a novel...
"The test of a round character is whether it is capable of surprising in a convincing way. If it never surprises it is flat."
- E. M. Forster
Imagine that the short-tempered businessman is the protagonist in your novel. When you first introduce him, don't be afraid to concentrate on his stereotype - in fact, play it for all it is worth...
The process of rounding him out will begin soon enough (in Chapter 2, in fact) but it is good to begin with a two-dimensional, yet vivid, portrait.
And so it continues...
As you show the readers the businessman's mean side, his sweet side, his ruthless side, his loving side, and so on, so his character becomes more and more rounded.
But it is not just through a complexity of character traits that three-dimensional characters are made.
When you tell the readers about a character's past, for example, and about what kind of future they see for themselves, that adds dimension to their character, too.
Flat characters exist only in the present - their pasts and their dreams for the future aren't mentioned. By giving a round character a complete life, you make them more of a complete person.
What could you say about the businessman's past?
Another thing which helps to add dimension to characters is motivation - specifically, the motivation behind their goal in the novel.
Let's say that our businessman's primary goal is to find his illegitimate son. The question that we, as writers, must provide good answers for is this: why does he want to find this boy?
Make it a little of all three and you will really have a round character on your hands.
Next Up: An in-depth look at Flat Characters...