Secondary characters fall into two camps; those who support the main character’s quest and bit-part characters. The former serve the main character arc, while the latter drive the plot and don’t occupy much screen time.
Although non main characters aren’t as fully fleshed out as the main characters, in terms of character arcs and goals, they must still be three-dimensional. Bit-part actors may often fall into a stereotype to facilitate a rapid assimilation with the audience. Stereotypes such as the street hooker or the over-critical mother in law, rely on subjective, widely-believed preconceptions about character type. Generalizing and forming patterns between people and their behavior are part of the human cognitive process in terms of who we like or dislike.
The secondary characters generally assist the main character in achieving their goal. In the case of buddy films, they act as a sounding board for the main character’s innermost thoughts. Sometimes, the “best friend” is the only person on the planet to tolerate the main character’s serious flaws before they see the error of their ways. However, they must be ever-present to reassure both the audience and the main character alike, that the main character is human and capable of being loved.
The secondary character also gives the audience an insight to the main character’s backstory. Perhaps they met in law school, jail, a nudist colony, war-torn Sudan, or an African safari. They also have an annoying habit of disclosing important aspects of the main character, often at the most embarrassing and inappropriate times, thinking that everybody already knew.
Secondary characters often cause a hiccup in the main character’s plan. This raises the stakes by creating conflict, but also frustrates the main characters, as they need to be reminded they have taken the wrong path.
If your main characters are primary colors in your painting, secondary characters are the shades to add texture, variety and interest to your script. You want the best actors to play these roles, so write them well.