Everybody knows about ball-joint dolls by now, right?
Okay, back up: Most of us grew up with Dolls For Girls (that usually meant Barbie) and Action Figures For Boys (which, when I was a kid, meant G.I. Joe). Barbie, with her clicky bendy vinyl-covered knees. The kid I was bent 'em fifty ways until one joint popped through the vinyl, revealing the structure underneath. Science! Then that leg was amputated, and half a ball-point pen made a dandy peg leg, and that same pen made gaps in her teeth and five-o-clock shadow and some tats, and Babs was launched on her career as the Scourge of the Caribbean. Modifying dolls is a thing that comes naturally to every kid.
Many doll traditions have as an article of faith that a doll is, or can be, a person. Go look up bunraku on youtube, it's one of the great classic styles of Japanese theater. Dolls don't have the same life as puppets, but there's still loads of character-building there... The good folks who make Asian ball-joint dolls, or BJDs, take dolls to a whole 'nother level. Like this, or this, or this. These dolls are terrifyingly expensive, exactingly sculpted, and cast in resin, which is light and very strong. BJD enthusiasts paint and repaint the faces and bodies, change out the wigs and eyes, and oh BOY, the clothes. Holy mama, the clothes. But the ball joints are what give these dolls a lot of their whammy. Remember Pirate Babs and her one remaining clicky knee? As movements go, she's not even on the same level as Lon Chaney's Frankenstein. Let's count her joints, or points of articulation: head turns side to side (1), shoulder swivel (2), hip swivel (2), bendy knee (1). That's six (but would be seven if it wasn't for Science). That's OLD Barbie, mind you; there's a world of Barbies out there now with way more articulation than that-- more on those later. But Barbie was made to strut down a catwalk. She looks good standing. Ball joint dolls do stuff like this and this and this. Points of articulation vary, but your typical BJD's neck joint is hemispherical, which means that one joint can swivel all kinds of ways. Your doll can look up or duck its chin, can tilt its head to give you the side-eye. Your doll's elbows and knees will most likely be hinged rather than ball joints, but that's not unlike the type of joint we actually have, so no big. The shoulders, wrists, hips, and ankles will be ball-jointed, which mean the legs and arms can swivel and turn with a great range of motion. There may be one or more torso joints, so your doll can tuck into a ball or flex into an arabesque. There are a minimum of fourteen points of articulation, and the types of joint used open up a world of my favorite thing, posture and gesture.
This is a fan thing, so there are clubs and cliques and rules and ideals, for what all that's worth. The resin-doll folks tend to stay away from the vinyl dolls, like Tonner and Fashion Royalty and Monster High. There are SO MANY fourteen-or-more jointed dolls out there, holy shmoley, and some of them are really awesome. People take off their factory paint and do all kinds of crazymazing things with them too. How about boil perms for natural hair? Or tattoos? Or just adding details to make a doll more what you imagined it would look like?
One inescapable difference between the average resin BJD and fashion doll is size. The resin dolls are bigger. Some of 'em are two feet tall! Fashion dolls take their lead from Barbie, who is a 1/6 scale doll-- this means two inches equal one foot. So, for example, a doll based on a woman who stands 5' 6" tall would be 11" high without bendy plastic platform shoes. Lots of resin dolls are twice that scale. This difference is not minor. The smaller dolls are fiddlier to paint. Their clothes are even harder to make, because the fabrics that drape and gather and stretch nicely on a person may not work on a resin doll and will look completely ridiculous on a vinyl doll half that size.
This is all going somewhere, I swear. My itchy itchy hand always wants to get into making people. Har-de-har, you thought I forgot about action figures, didn't you? The folks who are way into modifying dolls are all into the skinny elfy anime characters, right? HA HA HA HA HA. More on THAT rabbit-hole later.