I recently noticed this outstanding custom action figure of Wesley Gibson (The Killer) from the comic book Wanted (soon to be a movie featuring Angelina Jolie). Sam Costidell was kind enough to upload some in progress photos, and explain how he created the figure:
I started with getting pieces laser cut as an internal structure - sets of 3 discs for the joints, held together with central pins, attached to lengths to give me a solid structure upon which to build my arms, legs etc.
I sculpted magic sculpt directly on top of these, building up the shape gradually and sanding back where needed. Vaseline was put onto the areas of plastic at the joints where I did not want the magic sculpt to stick.
Once the basic figure is complete (as in, no pockets, armour etc) elements like the shoes, seams and creases were carved in or built up accordingly. Finishing the lower layer of the costume first is necessary to keep the finished product clean looking. This includes getting it to as smooth a finish as possible (keying can always be done later if needed for extra parts).
Once this was all finished, details like the pockets and backpack were sculpted onto the figure. These were done in stages, getting the basic shapes done before adding pocket flaps and buttons etc later. Each part was sanded smooth before anything extra was added to it.
The hands were sculpted around the gun handle, which was coated in vaseline to stop it sticking. The middle, ring and little fingers were done first, then the forefinger and finally the thumb. Most of the detail was very carefully carved (or rather, sanded) rather than sculpted.
The head was sculpted in sculpey firm - this allows you to work into it over a longer period of time than you would be able to do with any epoxy putty, as the epoxy will cure long before youre finished.
There was a minimal amount of cleanup to do after the sculpt was finished, then each part was isolated, primed and painted seperately. Black satin spray was used for the majority, but certain parts were picked out with gloss varnish or citadel paints.
Click through to see Sam's DeviantArt Gallery, including a very cool Gunship.