Other than a few sections where Jack Burden talks about himself in the second or third person, All the King's Men is strictly first person. Jack is both a central and a peripheral narrator. He tells us, "This has been the story of Willie Stark, but it is my story, too" (10.656). Within their two stories, we also get two other stories: the story of Cass Mastern and the story of Judge Irwin. But we also hear the stories of the other people closest to Jack and Willie. Since Jack can't tamp down his inner "student of history," he becomes a filter for these different stories and is compelled to preserve them for future generations.
Even though Jack is dedicated to finding "the truth," he still isn't the most reliable narrator in the world. As we discuss in "Writing Style," the whole narrative is a collection of layered flashbacks and imaginings. Memory is notoriously faulty and unpredictable. Plus Jack has heavy-duty baggage. He's carrying around trunks and suitcases of bitterness, anguish, shame, and brokenhearted-ness. At times, he is too emotional to be a reliable narrator.
Especially, perhaps, where Willie is concerned. Even when Jack likes Willie the least, he seems to hero-worship him. It is clear that Jack places Willie on a pedestal when he talks about him to others, and when he recalls memories of the Boss. At the same time, Jack is so involved in his own story that he doesn't quite penetrate to Willie's depths. In some ways, Willie remains a character in a story, while Jack seems more like a real person.
When looking at the other characters in the book, we should remember that we encounter them through Jack's perspective. As much as we adore Jack, we have to admit that he can be an unreliable narrator, who doesn't always have things right.
We should add that there is actually another first person narrator in the novel – Cass Mastern. Much of Cass's story, as told to us by Jack, is quoted from Cass's journal, which is written in the first person.