Sophisticated storytelling like that found in Sicario requires a sophisticated understanding of story. One can't rely on the idea that the central character of a narrative is also the one responsible for driving it forward. It certainly makes it easier on the writer to set the story up that way—but it is by no means an absolute requirement.
One of the things immediately apparent with Sicario is its exemplary structure whereas the main character is separated from the typical protagonist function…the main character serves as the perspective the audience sees the story through whereas the protagonist drives the plot – a concept many still seem in the dark about. Here the audience identifies strongly with Kate because we know just as much as she does, when she does, as the truth is slowly revealed. We are in her shoes and experience the story vicariously through her vantage point as part of the story’s design.
The idea that the alternative perspective to Kate's comes from both Alejandro and Matt is an interesting one; in my analysis I had seen Matt Graber (Josh Brolin) as more of an objective character (perhaps Contagonist) and Alejandro (Benicio del Toro) as more of the emotional counterpart to Kate. I feel like Matt plays more into Kate's personal issues and Throughline, rather than offering an emotionally significant perspective for her to consider…but I can see how there could be a handoff of persepctives from Alejandro to Matt when the hitman is out on his mission.
That is the beauty of understanding story through the Dramatica lens: the meaning of the storyform is more important than any strict rule of structure. The Influence Character perspective is simply that—a perspective—and as long as that perspective comes from the same point-of-view and challenges the Main Character to change their ways…well then, it functions appropriately within the Author's argument.
Whether simply Alejandro or a combination of Alejandro and Matt, the argument—and storyform—remains the same and carries the same emotional impact.
In addition to creating conflict on multiple levels, great storytelling is also able to present an argument and persuade its audience, challenging them to change their own beliefs over the course of its unfolding. In Sicario, we’re thrust into Kate’s perspective and are asked to see things as she does – more so because most of us share the same values and beliefs in the rule of law and a just system. It’s through her that the author presents their moral argument along with a separate viewpoint that is, more or less, diametrically opposed (a concept previously discussed here). The more details that come to light, the more we question our own beliefs and values as seen through Kate.