One of the worst offenses a writer can commit against their stories (and, by extension, their audiences) is to de-emphasize the characterization of their secondary characters in favor of only lending development to their main protagonists. Writers often make the mistake of taking their secondary characters for granted by glossing over their motivations, neglecting to give them multidimensional traits and qualities, or making it clear that they exist only to serve the needs of the protagonist or the plot. Here are some things to keep in mind while character-mapping or writing your secondary characters:
1. Consider the character’s goals and motivations: Here’s the thing about secondary characters: they shouldn’t behave as if they are aware that they are secondary characters. Each character is the protagonist of their own story, and as such, they have to be given the same level of consideration. If the character is helping the protagonist to achieve his ends, then what is their motivation for doing so? Self-preservation? Do they possess an honorable nature and a strict moral code of devotion and loyalty to their friends? Do they see any other benefits for themselves, such as earning riches or achieving fame and admiration?
The secondary character doesn’t have to outright state their motives, but the reader should be able to discern these motivations through previous interactions or scenes that establish who this character is. In other words, the audience should never be left to think that “because…protagonist” is the secondary character’s motivation for helping the protagonist in his or her arc. The secondary character must have his or her own reasons for any action they take. This also goes for secondary characters who act out of character just to cause unnecessary conflict in the plot: the audience will immediately pick up on the inconsistency because the character’s actions don’t fit their previously established motivations (or they have never been given any motivations beyond “because…protagonist,” which would make their sudden disruptive behavior even more illogical). In Captain America: Winter Soldier, for example, Natasha Romanoff doesn’t just help Steve Rogers because it’s convenient to the plot; she does so because of her moral code (as we saw in The Avengers, she believes very firmly in paying off her debts when she feels she owes someone for their deeds), and out of a sense of duty to Nick Fury.
2. Dimensionality: Have you ever seen that character in a book, film or television series whose sole purpose in the universe is to offer some levity in tense situations? This character is not-so-lovingly referred to as the “comic relief,” and he or she becomes problematic for the narrative when humor is the only trait and purpose he or she has been given. There are several other “types” in fiction who can be damaged by the pitfalls of one-dimensionality: “the love interest,” “the bully,” “the sidekick,” just to name a few. To reiterate, the problem with these characters is not that they fall into a type or trope, but that they are given no further development beyond that trope. You can have a love interest who also has a purpose beyond being the protagonist’s object of desire; take Lupita Nyong’o’s character, Nakia, in Black Panther, who had her own goals and interests outside of T’Challa, which allowed her to be a fully-developed character instead of a cardboard cut-out.
Part of establishing multi-dimensionality in a secondary character is, as mentioned before, giving the character their own aims and goals. Another way to establish multi-dimensionality is to give a character more than one attribute. Think of the people you know in real life: no one is ever just one thing. The funniest person you know may also have a nasty habit of jealousy and self-doubt, and they might be fiercely loyal. Or think of your own love interest: surely you have developed an emotional connection with this person because of not just one trait, but the entire amalgamation of traits, both positive and negative, which formulate who they are as human beings. The same should go for any secondary character: they should possess a realistic amount of attributes and weaknesses to fully humanize them.
3. Agency: As stated before, “because…protagonist” is not a legitimate reason for a secondary character to do anything, let alone for them to base their every decision or action on. No character’s life should revolve around the protagonist’s just because they’re the protagonist. A character who is only there to serve as the protagonist’s cheerleader, to do their bidding or to follow their every whim, is a character who lacks agency.
Your secondary characters can of course have moments of selflessness, but should also, for the most part, do something because they think it will help them to achieve their own goals, or because it fits into their morals. Brienne of Tarth in Game of Thrones, for example, follows her code of honor against all odds and, at times, against her best interests; because this has always been an aspect of her character, her actions and decisions make sense within the context of her moral code.
When it fits their character to do so, they should also act independently or even counter to the protagonist’s desires. If you have built a multidimensional character who possesses his or her own motivations and goals, then as long as their actions are in line with those aspects of their character, you will be giving them the agency to affect the plot and the protagonist, as opposed to simply serving the plot and the protagonist.