Many amateur or inexperienced fiction writers struggle with character development. They rely on expository description that tells readers who the story's characters are rather than allow the characters to emerge from dialogue and action and evolve into three-dimensional personalities.
All too often in attempting to impart an image of the character to readers, amateurs spend too much time and effort trying to transfer the image in their minds to the brains of their readers. Really, beyond anything truly distinguishing such as a distinctive scar, eye color, hair color, height, there really isn't much use in getting too specific. After all, if you search for "blonde, blue-eyed male movie stars," IMDb will bring up a list of actors who have those features, but look completely different: Brad Pitt, Chris Hemsworth, Chad Faust, Nick Carter, Aaron Carter, Bret Michaels, Justin Hartley, Ryan Phillippe, Ryan Gosling, Daniel Craig, etc. While one reader may picture Brad Pitt, another will imagine Daniel Craig in the hero's role. The writer can direct the reader's imagination in the desired direction to convey the intended image with a simple analogy: "She looked like a young Katharine Hepburn."
Further expository sins occur when the author dumps background information. Amateurs generally commit such sins at the beginning of the story or whenever a new character is introduced. Such description often reads like a resume, the dry biography concluding a scholarly paper, or an obituary. We learn that the character has three sisters, grew up in a suburb of Chicago, earned a degree in computer programming, drinks her coffee with cream and sugar, and works as a waitress; but, we knowing nothing about the inner person that speaks to the author.
Of course, sometimes a simple, declarative sentence gives more insight into the character than an entire page of description, such as the comments sprinkled through episodes of the television show Frasier. In the show, Niles' wife Maris never appears, but comments like the following give viewers a sense of the character: "Maris is like the sun. Except without the warmth."
The very best way to imbue your character with personality is through a combination of dialogue and action that show the character being funny, cruel, savvy, clever, kind, etc. There are ways to do this. For instance, the first sentence of "Skeins of Gold" conveys a wealth of resentment and perception on the part of the protagonist: "It’s the lot of women to suffer the stupidity of men."
Character development goes beyond physical description and background. Written well, it brings the character to life and connects the reader to the story. Effective character development makes the reader care for the character. It helps propel the reader through the pages and contributes to a sense of loss when the story ends, because the reader must then bid that character goodbye. Effective character development forges an emotional bond between between story and reader.
Sometimes character development lies more in what the author does not write. When summarizing character development, I fall back on art: "Think Impressionism, not photograph."