When I say “intimacy”, I mean the author needs to have an intimate knowledge of his point-of-view characters.
In real life, there is a part of us no one truly knows, no matter how close they get to us. Sure, people around us will have varying degrees of intimacy with us. Our spouse will know us better than our friend, and our friend will know us better than an acquaintance. But the only one who truly knows us is us.
The same should be true if we are to write the emotionally engaging character. We should know their backgrounds, their darkest secrets (even those that never show up in the story), and we should know their futures.
This can be a sad thing. I write romance, which is a happy genre. You always have a happy ending. But I know things about some of my characters that have made me cry, even though these things have never shown up in the book. I don’t write this stuff down. It’s just stuff that comes to me when I’m in bed or taking a shower or going for a walk. I can’t shut this off. I just “know” this stuff about the character I’m currently writing.
Usually, this is backstory stuff I had to take out during the edits. For example, there was a lot more to one of my character’s story that I never touched on in a book already charged with a depressing backstory. None of it belonged in the story, so it was never added, but it did add layers to this character’s life that made me understand her intimately.
But sometimes, this is something I know will occur in the future (long after I type “the end”). For example, I know one of my couples will lose one of their children to an illness. I’ve been asked to write more of their story, but I will never do it because I don’t want to make them “go through” that pain. As long as I don’t ever write it, they’ll be suspended in their happy ending. Believe me, the reader does not want to know more about this couple. It would ruin the book for them.
Our main character should be as complex as we are. I like to think the reader should be at the level of spouse/best friend with the character. But, the author is the only one who can look into the soul of the main character and see what’s really there.
The key, of course, is how much you reveal in the story. My suggestion is to only put in what you need in order to make the story the best it can be. Any revelation you give should advance the plot. If it doesn’t advance the plot, it doesn’t belong there.
So don’t be afraid to explore all aspects (good and bad) about your main character. You don’t have to include everything you learn. Just the parts that matters to the story.