So you have made The Stoic, and you have made him well. Not a crease or indent ever forms when the action starts around him and the blood starts flowing since he's such a tough, battle-ready anti-hero (and possibly because action stars don't make the best emotive actors). The problem is, what do you do when you need to show off that emotion? When the Big Bad comes to your hometown, burns it to the ground, enslaves his family, kicks his puppy and leaves his girlfriend's corpse inside some sort of chilled food storage device, do you make him explode with rage and potentially lose his badass image to your lead's Narmtastic acting or just leave him standing there with the same implacable look on his face, leaving the audience unconvinced that It's Personal?
Crck-crck-crk-ckchk
A series of short sharp crackling noises and a close up of the hero's fist shows you that his Kung Fu grip has gone into overload, and that his anger has manifested itself not into pointless words, but into preparing his fist for the hard beatdown. Wow! Before he wasn't even trying hard enough to stress his joints, but now he is willing to clench! And he's not just using his hands, either; his neck, his shoulders, his knees are all giving off threatening sounds as he works them. The sound released has nothing to do with the bones themselves, but rather with the joints between the bonesnote ; as such, any joint on the body is ripe for a satisfactory cracking. You're not actually damaging the joints or bones significantly by doing this, and it does not increase the chance of having arthritis - contrary to an old piece of "advice".
However, there are a few different reasons why you might not want to use that particular. First is if you're going for realism; the knuckle-cracking action provides increased looseness and flexibility of a joint for a brief period, at the cost of a slight cumulative reduction in the joint's strength - crack your knuckles and you get a weaker grip, so if the grip is important to what follows, the crack is counter-productive. And some people find the sound a bit Squicky rather than satisfying, so if you, your actor, or your sound effects artist is one of those people, you shouldn't do something displeasing to them. But what alternatives are there?
Crckcrkcrrrrrrrrnch...
Your hero grabs something from the environment, and crushes it in their grip. Different audio, same effect - a show of strength, and an intimidating sound to let his opponent know that he's serious. If it's still solid, he can promptly throw it aside for an emphatic clang; if it's left in pieces, he can open up his fingers and let the fragments fall. Obviously, the effect in this case is proportionate to what is being crushed; the deformation of a metal item can show off his Super-Strength, whereas balling up a piece of paper comes across more like Tantrum Throwing.
Subtrope of Kinetic Clicking.
