Skin to Skin
A large part of being successful when writing fiction is the ability to slip into character. Ideally, we should become each viewpoint character. Sometimes writers make the mistake of being more like a camera looking at the characters. Or they pop into everyone's head and tell you things about every character (omniscient viewpoint). There are times when these tactics can work well.
However, 98 percent of the time, writers do their best work when they "become" the viewpoint character and stick with one character per scene. This means that they feel, see, taste, smell, think, and know only what that character feels, sees, tastes, smells, thinks, and knows. In this way, the reader is immersed completely in the story. Ping pong viewpoint -- switching from head to head -- can be jarring and keep the reader off balance. Sometimes the reader has to backtrack to figure out who's who and what's what. It can be that confusing.
Writing in first person helps writers stay in one viewpoint and not slip in and out of a character's head. It's a good device for that and it's a popular viewpoint for many romance novels (ala Fifty Shades of Grey). Third person viewpoint is trickier, but I like it because it seems more like a novel rather than a diary to me.
Whatever viewpoint is chosen, the objective should be to use it to make your readers live along with your characters. I like to think that I can pull them in so far that, should a doorbell chime or a phone ring or someone speaks to them, they startle. They're so deeply into that fictive world that they forget everything around them. I know that as a reader I love books that suck me right in so that I'm oblivious to time passing and what's going on around me.
The same thing happens when I'm writing. Once I get inside my character and surrounded by the scene transpiring, I have no idea of the time. Often, I come up for air, so to speak, and I'm shocked that two or three hours have gone by. That's when I know I've been on a roll.
When I was learning the profession of novel writing, viewpoint was the thing I tackled first and it wasn't without struggle. Slipping into someone's skin -- even someone you've created -- is no easy feat! But it's so worth it.