One style that’s common to all genres is the double-act. From comedy to fantasy to police dramas, having two main characters is a powerful tool for increasing the tension and driving forward the plot.
One type of double-act takes two characters who are fundamentally different and observes what happens between them. In the 1987 film Lethal Weapon, the veteran Murtagh sees the world very differently from the trigger-happy Riggs, and they often fall out over each other’s actions.
However, a double-act doesn’t necessarily need to argue all the time. In Good Omens, Aziraphale and Crowley represent good and evil respectively, but they have a longstanding agreement to let the other do his job without interference. I find it interesting that the novel was written by a duo, but because Neil Gaiman and Terry Pratchett write in a similar style, I think that accounts for the consistent quality of the writing.
But even a duo needs support from time to time, and that’s where a supporting character can be useful.
In the Channel 4 comedy Peep Show, the duo comprises the serious Mark and the laid-back Jez. Two of their constant supports are Mark’s love interest Sophie and Jez’s acquaintance Super-Hans. Their actions can affect the two main characters, and drive forward the plot, in ways that wouldn’t have happened otherwise.