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?
A series of short sharp crackling noises and a close up of the heroes 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!
Sometimes the knuckle cracking sound is removed, maybe for the sake of realism, maybe because some people find it too squicky and then it will be replaced by the hero actually crushing something in his hand. Bonus points if its made out of metal, points removed if its only a paper note.
OK, great job. Your hero is walking around and he sure is pissed. He's walking at a constant pace to fight The Dragon who killed his old master and then vent his fury on the Big Bad in the ultimate showdown. Oh hold on, there are some lowly mooks in the way, oh it's even worse than that, it's just some random gang who's getting in the way. Well geez, how are we meant to play this for drama? The guy has already gone through swaths of baddies and gotten all the best equipment, they can't be a threat, we've got to hold the Badass Boast and speech for the top fight but the audience will get bored if he kills a man with one punch and then keeps punching him anyway.
Well, it did work the first time...
He's cracking his knuckles, he's cracking his neck, his shoulders, his knees, he's even making a few stretches. Aha, this time he's unimpressed. You may not tell it from his face, you may not have realised that thing on his face was a smirk but now he's so unworried by the oncoming fight, he's taking time to get comfortable first. You can even go one further and hold off the cracking until after the fight, that shows he never even had to worry about being uncomfortable even while unleashing a flurry of judo chops and kung fu kicks.
It might be useful to know that the sound released has nothing to do with the bones themselves, but rather with the joints between the bonesnote , and that you're not actually damaging the joints or bones significantly by doing this, and does not increase the chance of having arthritis - contrary to an old piece of "advice". What it does is provide increased looseness and flexibility of a joint temporarily following a crack, at the cost of a slight cumulative reduction in the joint's strength - crack your knuckles and you get a weaker grip.
Subtrope of Kinetic Clicking.