Clearly define their role in the work

The reason most characters’ stories go on for longer than they should is because the writer has not established a clear reason why they should appear in the story in the first place. The role of the supporting character must first be tied to that of the main character. They should not exist on their own as they are not the key drivers of the plot. Identify ways that you can create a link between the main characters and the supporting characters. Here are a few ways that supporting characters can add to your main character's role in the work:

Make them a part of your main character’s community of friends. No one exists by themselves. Your main character should have a few friends around them, even if the relationship may not be a close one.

Create a supporting character on the side of the antagonist. Most villains do not act alone. They also have someone or people that help them in their mission. Adding a supporting character on the antagonist’s side will give them depth and make their role in the story more impactful.

Make them the victim. In mystery novels, it is usually up to the protagonist to solve the mystery. The villain is the perpetrator but there needs to be a victim first. In this case, you can use a supporting character to create a link between the hero and the antagonist.

Other minor roles supporting characters could play are adding humor to the work or revealing something about the background of the story. For instance, if the main character is living in a crime-ridden place, the people he meets on the streets, the conversations he has with them or basically what he observes could create the background you are going for in the novel.

Make them real too

The realistic appeal of characters is usually emphasized mostly in the case of the main characters. However, making your secondary characters believable is also important to the story. They should have their flaws which you could reveal through the main character’s perspective. In the case of the villain, do supporting characters on this side of the plot support the villain’s actions? Is there one person who is hesitant to support them? For the hero, have they always shared a great relationship with their friends? How did they meet?

You can make the supporting characters real too by showing their failure in carrying out their intended roles. For example, if the supporting character is best friends with the main character, they could fail to show up when needed. They could also oppose the main character’s decisions. Attributes that will make them appear human should be included to create an unpredictable story and an interesting one.

Their roles should still be clearly defined. Just because they have a few weaknesses does not mean they cannot be on the side of the protagonist and also just because some on the antagonist’s side appear to be humane does not mean they will always act according to their conscience.