As well as a main or main characters we also need to have secondary characters. These are the characters that people the main character’s immediate world, they reoccur, sometimes becoming main characters, sometimes having been main characters who’ve moved out of the main story. For instance, in the first season of Buffy the Vampire Slayer we have Giles, her Watcher, as a main character helping to guide her. Later on, when Buffy is older and more experience, her Watcher/father figure is less prominent and eventually reoccurs but his role has changed because Buffy has changed so their dynamic is different.
Having seven seasons we can clearly see characters appearing as bit parts or secondary characters then joining the main cast then moving away from the main cast and becoming secondary characters, bit players or sometimes disappearing all together. Long running television shows are often the best way to see this change because of their visual nature, even if we don’t watch them we can see the change in the cast simply by comparing pictures from each season.
Secondary characters can appear anywhere from as often as main characters to periodically but they usually impact the narrative when they do appear. They might provide information, skills, a different emotional angle or a variety of other things depending on the characters or situations. For example, if we look at the Bond films then James Bond is definitely the main character then we have: M who gives him missions, Q who provides the tools for his missions, and Moneypenny whose role changes depending on which era we look at. There are also secondary characters who appear outside the headquarters such as Felix, a US operative, who doesn’t appear in all the films but is often important when he does often providing information or assistance. The first selection of secondary characters appear more often because they’re associated with Bond’s workplace, he often has to go to headquarters, sometimes just at the beginning of the story and sometimes at multiple points, which increases the likelihood of them appearing. Whereas Felix appears in the field, which can be anywhere, as he’s not associated with a fixed location or anywhere Bond is likely to go on a regular basis he isn’t guaranteed to appear on a regular basis.
We follow Bond’s adventure and he drives the story forward but these secondary characters all provide things the narrative would falter without. We could take Q out but Bond would have to get his gadgets from somewhere, either we’d have to give him new skills to create his own or we’d need a series of bit part characters to fill the void. Having one character in the form of Q allows Bond and the audience to develop a rapport with Q. In one of the recent films, Spectre, Q is put in danger and in theory the audience should be on tenterhooks as they worry whether he’ll escape (this really depends on whether the audience like him or not). There’s an entire sequence based around the risk to this character the audience may have develop an attachment to. If it was a random character put in danger then the audience are less likely to have an intense emotional response. They might want them to escape but bit players in Bond have a tendency to perish so if they do it’s less unexpected, it’s part of what happens in Bond.
Overall our secondary characters are going to reoccur and readers/the audience are likely to develop an attachment to them. They also serve a function in the narrative beyond popping up and disappearing without ever being seen again. If we can easily remove a character from the story without changing the main character’s world then they’re more likely to be a bit player than a secondary character.