Great actors know to dig into every piece of information on the page to flesh out their characters. Each character and scene description, each statement. And writers must do the same. Nothing you put on the page should be taken for granted. And, in fact, you must take those tidbits and expand on them. After you have planted the seeds of a character, look to see what will grow.
In an informative Q&A, Matthew McConaughey describes how he develops his characters. He sees each scene in a film as a quick look into an ongoing life. It’s as if the camera has caught a fleeting glimpse of a character’s ongoing journey. Matthew notes that the character has been and is going other places. He has a history and a future. Imagine what this might look like? What is the prequel and sequel? What are the character’s beliefs, needs, obsessions. Matthew wants to know what the character says to himself before he speaks aloud to others. What is the inner monologue? This will help us put very realistic dialogue and actions on the page. When we ourselves visualize the character as a fully developed person, it is much easier to show who they are through our writing.