What conditions must be met to trigger the knife animation?

I have been having a lot of trouble getting the knife to successfully trigger the animation in multiplayer, instead it just stabs in the direction of the person (doing 33% damage every time, it seems). Obviously, whenever I fail at knifing someone it tends to end badly for me, so I would like to figure out what I'm doing wrong.

What conditions does the game check to trigger the knifing animation as opposed to a knife attack?

According to the Battlefield Wiki:

Holding the attack button and approaching an enemy will perform a fatal Takedown attack. The takedown can be performed from the front, but it is potentially more difficult to achieve against an alert enemy.

Additionally, this may be helpful:

The knife has a special little feature for new users, if there is no target, or target is not in range, then the knife points vertically down when equipped. However when the target becomes in range the knife often moves to a horizontal position to indicate that the target is within range, this makes it much easier for newcomers to know when to knife (the range for selected knife attack is greater than quick knife, so do not press quick knife and remember that in a knife fight).

Holding the knife button (F on PC) or drawing your knife and then holding mouse click will perform the one hit kill knife animation. Tapping the button anywhere from behind (a 180 degree arc around the back of the player) will also perform the animation.

A few things to note:

• The takedown can be performed on both crouched and prone enemies

• You must be on the same level (not in terms of crouching/prone, you can knife a prone target while standing, I mean if the ground you're on is at a different height than the ground they're on) as your target. If you are above or below your target, you will just slash with your knife.