When you begin plotting or developing a story idea, the main characters are necessarily the focus, however, once you are into the details of the story it’s important to alter the focus occasionally to better develop the secondary characters.
If a secondary character seems to be falling to the wayside or not sticking in the reader’s mind, that can be a good time to turn the focus toward that character. This can provide a needed break from the main storyline and give the reader a change to process information or a big event or change while still progressing the story in an interesting way.
Shining a light of a secondary character’s motivations and desires may also give insight into his or her relationship to the main characters and overall storyline.
Consider questions of this nature to help you give a secondary character more meaningful page time: Who does this character love or hate and why? What does he or she fear? What are his or her thoughts about the journey taking place in the story? What personal issues of a secondary character may impact the main character’s journey?
It’s okay for a secondary character to have a life away from the main character. Most of their page time will relate to their interactions with or impact on the main character, but not all of it. Allowing a secondary character to exist semi-independently in the story provides more opportunities for that character to influence the story and main character because he or she is more fully developed.
A great example of a secondary character with an independent story that is only occasionally given focus (to great effect) is Cliff from Once Upon a Time in Hollywood. The viewer never gets his full backstory, but it certainly exists and seems to be quite interesting. It led to his connection and interactions with the main character, but it’s only brought to the forefront of the story when needed to explain why he sticks around and can’t find consistent work elsewhere in Hollywood. Even thought the viewer never gets to know Cliff fully, he feels quite complex and realistic.