After thousands of hours in the FO games I'm finally giving a full melee build a go. No ranged at all. I'm level 13 on Survival and loving it. Basically a stealth ninja build. One thing I'm not fully grasping though is why at times using a knife I get the special animation where you quietly assassinate a human enemy and other times I dont. Is this just a random flavor thing or is there a condition that needs to be met. I've been experimenting but it seems random.

Random flavour with normal attacks, but you can trigger it more consistently by using power attack instead of regular attack.

Cool, thanks. I thought it might have some significance because it seemed like when it happens I would remain hidden as opposed to caution. I rarely am using power attacks out of stealth. Almost always VATS because I generally need to clear a whole room in one go. Even lowly raiders chew up my health super fast if I go toe to toe with multiple ones. Went with a strength, agility build with only 2 endurance.