There are many different viewpoints from which you can tell your story:
In this viewpoint you can jump into the head of every character. Although omniscient viewpoint is considered the most natural one for storytelling, it does pose problems.
Because it isnt a good idea to tell a story from too many viewpoints (it can make the reader feel distanced from both the story and the characters) its best to reveal the thoughts and feelings of minor characters through action and dialogue. However, if this isnt possible, an occasional look into a minor characters head wont be noticed by the reader if its well handled. If shifts in viewpoint are noticeable its usually called head-hopping.
In omniscient viewpoint, its all too easy to change viewpoint in the middle of a paragraph, or even the middle of a sentence. Editors dont like this. Keen-eyed readers notice it too.
... she could only shout hopelessly as loud as she could, in frustration, expecting no answer. Lyo! What? he said beside her. Her shout turned into a scream; she seemed to levitate before his eyes. He bent quickly to pick up the mussels she had dropped. She came back down to earth finally and glared at his shaking shoulders. From The Changeling Sea by Patricia A McKillip
(She seemed to levitate before his eyes abruptly switches the viewpoint from her to him and is very disconcerting.)
The main difficulty here is that the narrator can tell the reader only what the I character knows. If, to heighten suspense, you want your readers to know that someone is stalking your narrating character, you have to go to great pains to give the narrator some clues and make him less smart than your readers (without annoying them; readers dont like stupid protagonists).
A second problem occurs only in books for children, because the narrating character is nearly always a child and needs to speak with an appropriate voice. This rules out any chance of writing with wisdom or poetic elegance. Even making your character a bookworm and top of the class in English doesnt work very well. You might get away with it if writing social realism, but fantasy needs special elements of style and wisdom to create that mysterious, indefinable air of magicelements that you cant use realistically with a child narrator. How would I deal with the problem? Well, Id probably start the book with a short scene that reminded the now middle-aged narrating character of what happened when he was 11 (or whatever age the character is meant to be) and give him a reason, even if just an overwhelming urge, to write down his story.
Ive yet to see a whole book written in this viewpoint, but Ive no doubt someone has tried it. It would be extremely difficult to pull off because it tends to have an unnatural, distancing effectat least it has this effect on me. However, I have occasionally seen a paragraph or two something like the following, usually in books written in first person.
You cant imagine what its like until it happens to you. Its as if a bomb hits you. You wake up to find the sun streaming through your window when it shouldnt have reached that far. That means youre late for school. And Mr Mitchells class is first today. Hell be furious. How come your mother forgot to call you? When you rush downstairs you find out. Your father has walked out and your mother is little better than a bawling baby. Theres no breakfast ready, no packed lunch waiting for you on the sink bench. You promptly forget Mr Mitchell. Even his worst punishments are nothing compared to this. Perhaps a cup of tea might calm your mother. Thats usually what adults suggest when someones upset. But the kitchen looks as if your parents were throwing things at each other. You have to fight your way through the mess to the kettle.
(In this example, the child narrator has used second personunconsciously suggested to her by the burst of bitterness that causes her to address the reader directly in the first sentencein an attempt to distance herself from what she is narrating. This is not happening to her but to you.)
In this viewpoint the story is told almost exclusively through the eyes of the main character. One of the biggest difficulties for beginner writers is staying in it. Its all too easy to slip into the omniscient viewpoint without realising. Because of this, beginners are advised never to slip out of third-person limited viewpoint. However, if the break is necessary for the story to work, theres no reason why this rule cant be broken. The story is always paramount, particularly in books for children. Just make sure your break doesnt jar (as in the example above). In writing for younger children its particularly important to stay within third person limited viewpoint as much as possible or readers are likely to skip until they find the main character again. (Even worse, they might put your book down and not return to it.)
Never break this viewpoint, however, if youre writing category romance. Publishers of this genre demand that you stay in the heroines viewpoint.
So if, on reading through your completed story, you feel it has an unfocused air, or the main character comes across more as a minor one, then look first for accidental shifts in viewpoint before changing anything else. Fixing these might be all your story needs.
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