The sniper peered through the scope. He made a minute adjustment for the wind that had sprung up in the last few minutes, and brought the crosshairs back to chest-high, centred on the front door. If the target kept to his usual schedule, he'd be coming out to get the newspaper any moment.
Behind the door, James Frost was pulling on an overcoat. The newspaper was at the end of the driveway, and now was not the time to get chilled, not the day of the board meeting. He opened the door.
The above example is abbreviated, created to illustrate the shifting Point Of View. Notice that the first paragraph is in the sniper's mind, while the second paragraph shows Frost's thoughts. The author is using an omniscient narrator. The narrator knows what both characters are thinking and doing. The effect is that the reader is standing beside the narrator, looking down from above, seeing everything.
A danger in using omniscient is that when the POV bounces from character to character, the reader remains distant. An author using an omniscient narrator has a harder job engaging the readers than one writing in third limited POV, where the reader is more intimate with the character. (In 3rd limited, the reader is viewing everything through a single character's eyes, instead of being told what the character sees by the outside omniscient observer.)
If you use an omniscient narrator, you must work hard to bring the reader closer to the character. For example, in the first paragraph above, the writer could have shown the sniper checking his watch, feeling exposed in his position now that the sun was rising, or wondering whether this would be the day a normally punctual target breaks his routine.
When the POV switches to Frost, the writer could have increased suspense by having him look for his shoes (delaying the deadly action about to come), wonder about what he was going to buy for his wife's birthday that evening (increasing our sympathy for the man), perhaps ask his son to go out and get the paper (raising concern in the reader that the wrong person was going to die).
Stories written in omniscient by beginning writers often read more like newspaper reports, with lots of telling but not much showing. Because the reader starts as an outside observer, the author has to take pains to bring the reader closer to the characters.
If you are using an omniscient narrator, don't try to create suspense by the hackneyed method of hinting about the future:
If she had known what was in the box, she wouldn't have opened it.
That was the last time he would see her alive.
Later, Julie would come to regret her words.
A further danger of omniscient narrators is confusion about who is giving an opinion. Consider this (written in omniscient):
Despite not knowing the sender of the email, Barb clicked on the link. Big mistake.
Was it Barb who realized the error, or was that the narrator's opinion?
Writers who use omniscient narrators have to use extra words, filter words, to distinguish between a character's thoughts and perceptions and those of the narrator. (See Filter Words.)
For example: Listening to his iPod, Tom was crossing the street when a car came around the corner.
Did Tom see the car? If you were writing in 3rd limited, Tom's POV, then the answer is yes. Everything the reader learns comes from Tom. But in omniscient, not necessarily. Perhaps it hits him before he becomes aware of its presence. To show us that Tom was aware of the car, an omniscient narrator would have had to say,
Listening to his iPod, Tom was crossing the street when he saw a car come around the corner.
"he saw" is a filter expression, needed by the omniscient narrator, but to be avoided by the 3rd person limited narrator.