Why not just drive the knife upwards through the base of the skull? Probably requires a good deal of strength, (and a strong knife) but should be instantly fatal.
Along the same lines, is the old piano wire garrote trick faster?
As an EMT, I treated two victims with slashed throats. Both survived and recovered. One was a suicide attempt with a very sharp knife that did sever the right jugular and nicked the common carotid. The other was an assault that avulsed a large flap from the right side of the neck below the point of the jaw, I didn’t find out what the exact vessel damage was. As others here have said, both wounds were extremely bloody. And although both were intended to kill, neither was fatal.
Of course neither was inflicted by a Delta Force/MI6/Super-special secret agent man like in the movies. 
Sounds good to me. THe only problem is the aiming factor. Too high and you hit the skull , too low and you might just glance off of the vertebae. Both would be painfull , but I dont think that either would be fatal.
If I were going to attempt something like this and wanted it to be clean and quiet I would try for a shot on the eyesocket. They used to do lobotomys with a spike at the top of the eye socket, so a long narrow bladed knife should have the same effect.
My other option would be to use a wide bladed knife and go for the spine either between the shoulders or just above the belt line. Either should sever the spinal cord and cause paralysis.
Reading the collective responses to this thread has just given me a serious case of the willies.
::shudder::
…So I think we’re up for a Dopefest soon at the old farm ten miles from the nearest town, you in?
Like this? 
Another vote for incredibly messy.
I was immediately behind a vehicle that strayed off the road and stopped abruptly in a culvert. I expected to find only minor injuries, at the very worst. I was wrong.
The injury was rather worse than asked about in the OP, none the less, it relates. It was horrible in gruesome. The amount of blood of startling in the extreme. I don’t think anyone really wants the details, but it was not especially quick, and it was horribly messy.
Hmmmm…I’ll agree with those that say “messy”, however, I assume that it is not that tough (regadring animals).
Long ago, when I was a teen, I was in my fathers boyhood village in Greece up in the mountains. My folks were using video cameras to document everything about the village that they could before it disappeared (modernized). My mom was to film the action at the slaughterhouse. I went with.
The butcher of the village was a huge, burly man. However, his 2 teenage daughters did most of the work.
The slaughterhouse was simply a large cement room with some hooks on the ceiling and a trough in the floor that led outside. There were about 4-5 goats in the building just kind of milling around. One of the girls grab and hold a goat (over the trough) while the other would take a large blade and slit the throat. They would then drop the goat to the floor. The amount of blood was enormous. Also, to me, it sounded as if the goat was trying to bray. Instead it came out as a guttural kind of bubbly groan. The goat would kick for a while and then stop. Meanwhile the other goats were flipping out in the far corner.
Anyway, considering that 2 small teenage girls were able to do this without much trouble suggests to me that slitting animals throats can’t be too tough. Oh yeah…they wore no gloves which freaked me out.
Something like that. More like aiming between the eye brow and the eyeball , upwards at a 45 deg. angle. Also note, before I get told that would be impossible , I said “Try” not succeed.
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- You can see for yourself. There are Chechen videos circulating on P2P and assorted “murder & mayhem” sites showing a captured soldier getting his throat slit alive. I cannot tell from the video if it was the Russian soldiers or the Chechen rebels doing the slitting. It doesn’t look too messy at first, but the videos are rather short, only a few seconds. Pretty nauseating stuff, far more disturbing than any fake-bullshit horror movie.
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- You can see for yourself. There are Chechen videos circulating on P2P and assorted “murder & mayhem” sites showing a captured soldier getting his throat slit alive. I cannot tell from the video if it was the Russian soldiers or the Chechen rebels doing the slitting. It doesn’t look too messy at first, but the videos are rather short, only a few seconds. Pretty nauseating stuff, far more disturbing than any fake-bullshit horror movie.
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Is there a Schochet in the house?
Don’t forget to include the clever quip. Whenever you kill someone, they should at least know how witty you are.
My dad, as a Green Beret (Army Special Forces) in Vietnam, knew a Navy SEAL with a scar running the length of his forearm, from attempting to slash somebody’s throat from behind and missing. The Army SF preferred a silenced .22 behind the ear; much quicker, quieter, and less messy.