1. With the knife moving in his hand, make sure it has his arm and hand move for this so rather then the knife magically moving, move his arm and make sure to add some realism to it but starting off slow with the knife movement to fast in the middle, to slow at the end, this is called easing.

2. Have his body move a bit more when winding up for the stab, maybe have him put his hand on the guy’s shoulder and have his feet spread apart a bit so as he stabs he doesn’t fall over.

3. When the knife comes out from the chest and he flicks the blood off, add some visual blood come off from the knife, make sure to ease the movement where he takes the knife out, then shakes the blood off.

4. Don’t have the guy with the knife cut away at the guy’s neck like a tree being cut down, one clean swipe would be enough to slice his jugular vein causing his death.

Across most animations, 25 – 35 is good.

I always animate at 32 fps.