This is hard to do, and it takes time, but it pays off with stellar story.
When you’re blocking out your scenes (we talk in detail about scene blocking at the Wolves level and up), BEFORE you even start writing them, get inside each of your character’s heads and act it out from their POV as if you were them, so literally...
GET UP AND WALK AROUND, AND MOVE AND TALK LIKE THEM.
Do everything you can to visualize how it would look for them—what’s happening in the scene. Then switch to the next character. And the next. And the next.
This takes time and is usually something you want to skip doing.
Think of this time as the investment in your book’s success. What will happen as you practice, is you’ll speed up. Soon you’ll be able to hop in and out of each character in moments. This is writing mastery. Until then, make an effort and do is slowly and deliberately.
I’m still slow at this, after 6 years of writing. It’s because I only started doing it consciously this last year, but the results are showing. So I can’t recommend this enough. Do it BEFORE you write, so you know how to control your characters and steer them in the right direction, not the other way around. When you act it out while writing, without having plotted it first, your characters will rule you and drag you into places where your story stalls, and it'll take forever to dig yourself out.