One of the most important story elements in any writing is: Who.
As mentioned in the previous post, The Lead, the first paragraph of a news story should at minimum describe who, what, where, and when. Unless, of course, there is a reason to add why and how in the lead.
Who is always a critical element. We all want to know who did what to whom. That is what we connect with, what we find important, what drives our curiosity.
Of course, the who of any story is the character or characters. The main character, secondary characters, and the antagonist are all included in the who. These are who the reader wants to know about, read about, get involved with.
So we make the character is as real as we can. Use of a character sketch (see an earlier post on just that topic) or character interview helps to fully flesh out details to make a character as real as possible. A character with depth is easy for the reader to settle in with for the duration of the story. To care about. To love. Or to dislike in the case of an antagonist.
The character sketch contains a lot of detail that may never make it directly into the story. However, you use it to add nuance and flavor to the words you choose when describing what the character does, says, and feels.