I don't know he'd get the knife through the breastbone at all to be honest with you. Maybe slide it between the ribs to the heart but.... Yeah. Killing someone with a knife is, I gather, not a trivial skill unless you're in a position to just slash/stab away to your heart's content.
A knife slash – while it may hurt and leave you permanently injured – seems unlikely to kill you unless you take it to the inside of the arms or to the neck. Somewhere an artery runs close to the skin. With the exception of some larger weapons, the general theme with a knife seems to be that you set up with the slash, using it to cripple someone's arms to a point where they can't adequately defend themselves, but you kill with a thrust to a vital area that's not too well protected by bone.
Thus, if your enemy is unaware of you, it is likely that it makes sense to get as close as you can without them noticing you, preferably from behind them, and make your first move a thrust, then back off and let blood loss do its work. There is an artery that runs a little distance under the top of the shoulders called the subclavian artery. If you find that area between the collarbone and the shoulder blade and drive your knife down through it, there's probably nothing that's going to get to the person in time to save them. Wouldn't require much leverage to hold them still and you can strike them in the throat with your other arm at the same time. If you strike someone in the throat with any significant force, you force them to inhale, preventing them from crying out.
The arteries in the neck I take to be well known enough not to require much explanation. A little deeper than one might expect but not dramatically so. Perhaps not a bad approach to stab into the side of the neck and use the knife as a lever to further open the wound if you've the luxury of position.
What'd be the other ones? Brachial artery I suppose. Follows the bicep and comes closest to the surface near the elbow. A deep enough slash there would sever the artery. Probably be a relatively slow death, so you'd have to be prepared to back off in a hurry or to control their movements so they can't strike you while they're dying, but sometimes you'd have the leverage on an arm to expose the inside and it might be a decent target of opportunity in that context.
Defensively? They'll probably be close enough that you've lost the chance to one up them on the weapon front - if they're not going to give you the option to back off an aren't just there to punish you. If you end up wrestling over the knife you'll probably get cut or stabbed. There are too many degrees of movement to restrain someone's arm easily when they don't need much leverage to do damage, and in any case they can change the hand holding the weapon.
Ideally, I suppose, you'd want to deflect the knife's path if they're following a line and use the opening that doing so creates to strike them hard in the head, or to drive their head into something hard (pavements come strongly recommended for this.) No other course of action that I'm aware of seems like it would be reliable. You'd need to take them out in the same beat that they'd be withdrawing the weapon for another attack – a hesitate and you're lost kind of dealio. If you end up getting cut in return for taking them out, that's not inevitable but it's probably a worthwhile trade-off to end the fight then and there.
If they came in in a wide arc, I suppose you could do a force on force block, in the same way that you might for a hook - but I don't see why they would. When I think of how I'd use a knife if someone was aware of me, I'd wield it in extremely tight curves that would prevent someone getting inside and allow me to rapidly withdraw the blade - or I'd go for a vital spot immediately with a very straight line, which would be vulnerable to the previously mentioned deflection idea. Someone would want to deflect it, try to strike back, try to get behind the elbow, or at least secure behind the elbow. Maybe trade a slash of an arm so restricted for taking you out.
Then again, I doubt too many people using knives have much training. You see people wind-milling all over the place in fights with their firsts, and I wouldn't do that either. Though at least that I can see why
they're doing it.
:/ Difficult to say on the defence side. It's much easier to imagine how you'd use something than how other people will.
That'd also be the reason why carrying a knife as a defence against other people carrying knives seems a bit iffy to me. If you show a knife maybe the other person with a knife will back down – but they also might not be thinking clearly and take it as a challenge they need to attack, and if you saw the knife in the first place chances are they were using it to force compliance, so just give them what they want instead. If it comes to a fight a knife won't guarantee you that the other person will be taken out before they can kill you. It's entirely possible that you stab him and he stabs you right back before he dies, especially if you don't know enough about fighting to set up for a position where he'd find it difficult to stab you back and withdraw out of range while he bleeds out.
Even swords are, by all reports, difficult to ensure a quick kill with unless you take someone's head off. There are stories of people being run through in duels, multiple times, and still going on to mortally wound their opponents. Of course the biggest killer back then was infection, so perhaps that's not too surprising, but you get the idea. You kill him, he kills you. Oopsie!
So, yeah, unless you have the advantage of surprise, I suspect
knives are pretty difficult to kill with - in a hurry anyway. It seems like you'd need something bigger for that. A machete might do it. But even then you're not ensured that they'll die before they get to cut you in return.
You might even be better off just bringing a trench club - a length of wood with a lead weight in the end - and braining your opponent repeatedly with that. I don't imagine that leaves someone too capable of defending themselves after the first hit.
There are other form of edged weapons. Long thin blades designed
to go in through the back and pierce the airway or the heart. A large heavy blade to the back of the neck would render a rag doll pretty much instantly. But ... in those cases you'd still not be opening the chest cavity up to stab the heart. That seems to me a total fantasy creation for the film.
It strikes me that knives are mostly tools, some of them are tools for killing I suppose - but with very limited tactical utility that's easily eclipsed by other options. That anyone trying to use one in a fight probably hasn't thought it through too carefully, or is hoping for the luxury of being the only one armed.
I mean if you knew you were going to get into a fight - and for reasons unknown, maybe ninjas kidnapped your family, and somehow guns and other more realistic weapons weren't available, and somehow you couldn't call the police... and so on. Why the heck
aren't you wearing body armour?
Choice between a knife and body armour. Oh, gee, I'll take the knife.
Yeah, good luck with that.