Haggis is right. Killing someone silently with a knife requires that you catch them completely off-guard and either cover their mouth with your free hand, and/or bury your knife in a spot that makes crying out impossible.

The old ear-to-ear cut makes saying more than "gurgle" more than a little difficult, and a quick thrust into the brain-stem actually is an instant kill, but any of these techniques requires more than a little skill, though I wouldn't say it's significantly more skill than successfully making a kill-shot against a target with a gun from more than 100ft; depending on training and circumstance.