One challenge for fiction writers is to see the world in multiple ways, simultaneously. Each character in a novel has their own world view and the author needs to be able to climb inside the skin of each of them. These world views will differ, just as each person is different, with different histories, perceptions, desires, and ways of understanding and explaining the world. Naturally these different world views, however subtle, will cause people to talk differently, make different choices, and often respond differently to the same situation. Each character is unique and the writer has to own and exhibit that uniqueness.
It is not enough to comprehend this intellectually. The best writers come to feel as their characters do, drawing on their own selves, like a method actor. This immersion requires a fluidity of personality, finding something of yourself, something genuine, within your characters. That can be exceedingly difficult if your character is say, a mass murderer. But you have probably experienced some of the anger and rage such a character possesses, not as strongly, but some version of the feelings that are relatable to your internal life.
Even first person storytelling requires that other characters be believable, behaving in ways consistent with who they are, and for that to happen, the author must know those characters intimatetely, deeply and realistically.
To be sure, that is a challenge because in the real world we are primarily one defined personality, with a unique and consistent foundation from which we act. But in literature, writers have to step outside their own frame of reference and turn into all their characters, however briefly.