How we change what others think, feel, believe and do
2D and 3D characters
The characters that appear in stories are sometimes described as 'two dimensional' or 'three-dimensional' (2D or 3D). The metaphor is that of reality, that a three-dimensional character is somehow more realistic, whilst a two-dimensional person is flat and relatively lifeless.
The two-dimensional character is simple and unexplained. They appear and they do things, yet you do not know them as people.
Stories can get away with 2D minor characters, but if major characters are 2D the whole story will fall flat and lack credibility.
We pass many 2D characters in the street every day, yet some people we can tell in a moment are much more than that...
The three-dimensional character is first of all believable. The appear as credible people who you might know. Like humans, they have flaws and failings. They are individual and also seek to relate to others.
The trick of creating a 3D character is to add detail that is not strictly necessary for the plot, yet which helps to create a sense of reality. The critical trick in this is to do it without losing the reader, who will quickly get bored if you go too much and too quickly into character development.
Just as you discover attributes about your friends across the time that you know them, so also can a character develop across a story. If you are writing a whole series, then they can develop further.
Just because a character appears briefly in a story, it does not mean they cannot tell a story of their own. Even by the way they dress, much can be told, from high fashion to down-and-out rags. When good actors appear in small and cameo roles they can easily steal the show with masterful demonstration of all three dimensions.
Here is a table that contrasts typical differences between 2D and 3D characters.