The reason 3rd is difficult to codify is because it actually covers a great deal of territory. So I'm going to build a spec for 8 different kinds of third, following a rough sliding scale from subjective (or tight third) all the way out to full omniscient, in 8 steps.

1. Pretend you wrote a story in first person. 'I did this, I said this, I thought about that.' Now change the every instance of I into he. The extreme tight end of the 3rd person scale has the story told from inside the head of a character exactly like that character would have told the story, if he were telling it, right up to being influenced by way that character speaks. (This is often called as 'tight filtered third', or 'subjective third')

2. Next you have a story that is told from inside the head of that character, feeling the characters physical sensations, but not using the same words that character would have used.

3. Now pretend you are telling the story from the point of view of a small camera that perches on the character's shoulder. This camera is equipped with a microphone that can hear thoughts, but it does not experience any of the physical sensations that the character does.

4. Next we get a camera without the telepathic microphone, poor thing.

5. Now our camera hovers a short ways away from the character and can look back at the character, instead of always being focussed on what the character is looking at.

6. The latest model of hovering camera is equipped with a telepathic mic with range enough to listen to the thoughts of anyone in the room.

7. The fully omniscient camera has full fly and soar capabilities instead of that pitiful hover. It also has telepathic mics and time-travel capabilities. It can see and hear anything.

8. Camera eight is not only fully omniscient but it comes with its own commentator who interprets what the camera sees.

Now lets try some examples to make sure everyone can recognize the 8 types in actual text form.

1. The blood was red. Bright red. It spurted out of her hand, and she stared at it in wonder. Shouldn't it be darker? Then the pain hit -- bright and hard and horrible. She gasped. Her breath rattled on the way in, and then she was screaming. The nurse flinched. Drew back from the noise.

2. Layla stared in stupefied amazement as the blood that blossomed from her hand in crimson droplets. 'Shouldn't it be darker?' she managed to wonder before the sensation of pain finally made it's way through her befuddled brain. It was a bright searing pain. She gasped in a rasping breath that somehow only went half way in before it escaped as a scream. The medical technician winced and drew away.

3. Layla stared in stupefied amazement at the blood that blossomed from her hand in crimson droplets. 'Shouldn't it be darker?' she managed to wonder before the sensation of pain finally made it's way through her befuddled brain and she gasped and then began to scream. The medical technician winced and drew away.

4. Layla stared in stupefied amazement at the blood that blossomed from her hand in crimson droplets. It seemed to take a while for her to even understand that she was hurting, but finally she drew in a rasping breath and began to scream. The medical technician winced and drew away.

5. Layla stared slack jawed at the blood that blossomed from her hand in crimson droplets, and for almost a second did not respond as if she was in pain, but finally her dull eyes tensed, she drew in a rasping breath and began to scream. The medical technician winced and drew away, and the nurse who had been waiting attentively just out of her line of vision, bit her lip and looked down at her tray as if to conjure up something that could stop the sound.

6. Layla stared in stupefied amazement at the blood that blossomed from her hand in crimson droplets. 'Shouldn't it be darker?' she managed to wonder before the sensation of pain finally made it's way through her befuddled brain and she gasped and then began to scream.

   The sound echoed off the walls of the room resounding in the ears of the medical staff who, already jittery from feelings of guilt and a slight but omnipresent fear of the law, winced and drew back. The nurse who had been waiting attentively just out of her line of vision, began to search for an excuse to stand attentively just a little further off, and her desperate gaze fell to her tray. 'A needle full of hartisca solution would shut her up' she mused, but reluctantly she rejected the possibility. The General wanted this one alert, and hartisca took forever to become fully metabolized.

7. Layla stared in stupefied amazement at the blood that blossomed from her hand in crimson droplets. 'Shouldn't it be darker?' she managed to wonder before the sensation of pain finally made it's way through her befuddled brain and she gasped and then began to scream. The sound echoed off the walls of the room resounding in the ears of the medical staff who, already jittery from feelings of guilt and a slight but omnipresent fear of the law, winced and drew back. If the room had not been so well shielded the sound might have traveled down the hallway and around the corner to where the General sat in conference with a small man in a dark suit.

   'Make this quick,' the general ordered. 'I've a prisoner undergoing a procedure right this minute, that I had intended to supervise personally.

   'Of course, of course,' the small man responded, smiling and oily smile that hid his inner disgust. 'I wonder what poor kid they are torturing now,' he thought as he pulled out his portfolio.

8. Layla stared in stupefied amazement at the blood that blossomed from her hand in crimson droplets. Because of the presence of Trityplitchin in her system it took nearly a second for the sensation of pain to make it's way through her befuddled brain. Finally she gasped and then began to scream, the sound echoing off the walls of the room, and resounding in the ears of the medical staff who, if they had less guilt on their consciences might have been able to withstand the assault with more serenity. Instead they winced and drew back, and the nurse who had been waiting attentively just out of her line of vision, began searching her tray for something that would stop the upsetting racket. Unfortunately for everyone involved, the General's orders prevented the nurse from reducing her charge to merciful unconsciousness.

   Well shielded from the pain of his devising, the General sat a short distance away in conference with a small man in a dark suit. 'Make this quick,' the general ordered. 'I've a prisoner undergoing a procedure right this minute, that I had intended to supervise personally.

   'Of course, of course,' the small man responded, smiling and oily smile, but if the General had known the hate and anger that roiled in that narrow chest he would not have leaned back so negligently as the sales representative pulled out his portfolio and spread its contents across the table.

So, how are we doing? Are we on clear the different varieties of 3rd?
Great! Now we complicated things by talking multiple viewpoint characters. :)

Think of it like this: Instead of using one camera with swoop and fly capabilities, we use more than one camera, but each camera is of one of the more limited varieties. (Type 2-4 usually.)

Now instead of swooping our omniscient camera over to the general and our little man in a dark suit, we do a camera change maneuver. You can do this in two ways. The most popular is a fade to black, or scene switch. Like so...

Layla stared in stupefied amazement at the blood that blossomed from her hand in crimson droplets. 'Shouldn't it be darker?' she managed to wonder before the sensation of pain finally made it's way through her befuddled brain and she gasped and then began to scream. The medical technician winced and drew away, leaving her alone with her pain.

[scene break usually indicated in ms form by three asterisks.]

In another room in the same building, Carstairs smiled his best oily smile at the General, who was leaning back in his chair. 'Make this quick,' the General said, 'I've a prisoner undergoing a procedure right this minute, that I had intended to supervise personally.

'Of course, of course,' Carstairs responded, forcing his smile to grow bigger and oilier in an effort to hide his inner disgust. 'I wonder what poor kid they are torturing now,' he thought as he pulled out his portfolio.

Many books using multiple tight third only ever switch cameras at a scene break. In order to do such a novel, you go through every scene, decide which character's camera you wish to tell that scene from, and eliminate any information not available to that particular camera.

It is also possible, however to switch cameras on the fly. You follow one camera into a scene where the other camera is present, and then you follow the second camera out.

In order for this to work you need to strongly cue the camera change, by starting a paragraph with something that could only be seen by the other camera. Ready?

Carstairs waved his gun at the various medical personnel around the room. 'Stand back!' he commanded. They shuffled backwards. 'None of them seem interested in causing trouble,' he observed to himself. 'Good.' He risked taking his eyes off of them to find a roll of gauze and hand it to the girl.

Layla was having trouble understanding what was going on. Her brain still moved very slowly. 'Is this a rescue?' she asked herself numbly as she grasped the gauze and began winding it around her hand. She watched the gauze turn crimson, tiny tendrils of blood rushing their way along it's fibers. But when she realized what she was doing she closed her eyes. She needed to think, she needed to be aware. The small man in the dark suit was talking to her now, asking her if she could stand.

Making a switch in the middle of a scene like this usually isn't a problem, as long as it is cued correctly. However, switching back to the Carstairs viewpoint in the next paragraph probably would be a problem. Frequent camera switching gives readers sensitive to camera position a feeling of whiplash. Once you have switched cameras you want to stay with the new camera for as long as you can.

POV change whiplash does not occur with a properly written omniscient camera, because the reader follows the camera as it withdraws from the head of one character and then enters the head of the other, as follows:

Carstairs waved his gun at the various medical personnel around the room. 'Stand back!' he commanded. They shuffled backwards. 'None of them seem interested in causing trouble,' he observed to himself. 'Good.' He risked taking his eyes off of them to find a roll of gauze and hand it to the girl.

She stared at him as if she was having trouble understanding what was going on. And he wished that he had more time to let her recover from her shock, but time was of the essence. He was only one man, and the room was filled with hostile gazes. The general's medical staff were none too pleased to have a gun waved in their faces, or their experiment interrupted. Furthermore, as they had no knowledge of the General's fate, a fear of his enraged reaction warred with their fear of Carstairs' weapon.

Layla's gaze had fallen to the gauze and she watched it turn crimson with unnatural concentration, tiny tendrils of blood rushing their way along its fibers. Eventually an awareness of the need for haste caught up with her, and she closed her eyes. 'I need to think, I need to be aware,' she told herself inwardly, and when her eyes opened again the gaze she turned toward Carstairs was no longer dull and lifeless, just stark with pain.

'Can you stand?' Carstairs asked her.

You will note that I try to go from the personal thoughts of one character, to impersonal depiction of those thoughts, to a hazy overview of the thoughts of exterior characters, to an impersonal depiction of a new characters thoughts, to an actual relaying of that character's thoughts.

So the difference between multiple changing on the fly third, and omniscient has to do with the way you change viewpoints. Do you ease your way out of one head and into another at will, or do you simply cut straight from inside one head to inside another, but do so infrequently, using care and discretion?

Personally, I don't think that 'multiple third is currently the fashion' is a particularly good reason to not do omniscient. I do think, however, that whichever POV you use, it should be applied consistency.