If you want to bend an arm or leg there are a few ways to do it. Dwights suggestion works but is tricky. Too much and it will melt, not enough and the plastic will distort as you bend it, not making a very realistic bend. I save the heat method for simple movements like spreading legs apart a little wider or slightly bending an arm or leg.
A slight bend can be made by cutting a wedge to the inside of the bend from the limb. Cut almost through leaving just enough to keep it attached. From here you can apply a little heat and bend to the angle you want. Once it cools it will harden up and you can fill the gap with putty and shape with a needle, scribe, piece of wire, etc (whatever will help you re-sculpts folds or clothing).
A more severe bend can be made by cutting the limb at the joint and then tapering the side of the limb to on both pieces to compensate for the bend. Drill small holes in the end of each piece and insert and glue a piece of wire. This will hold the pieces and strengthen the connection. From there you can build up and sculpt the missing material to "fix" the break. This will make for a well supported connection and if there is a small gap created by the kerf of the saw or knife when you cut the piece or trimmed it, the putty or epoxy putty will have something to act as a frame work to build around. By connecting the two pieces of limb with the wire, especially if you use softer brass wire, you can re bend or fiddle with the joint angle till you get it just the way you want it. Once in a position you like, you can use your filler to sculpt out the rest.
This animation will come in handy for more involved projects as well as simple changes to make your figures more natural in their setting. Hands, feet, heads can all be sawn from their bodies and turned or angled to better fit the scene. One thing I tend to do alot of is to cut the hands from my figures and hollow out the sleeve. I then attach a piece of wire to the hand and drill a hole up in the sleeve for the wire. This not only lets me mix and match hands, but I can change angles as well as give my figures that added touch of detail rather than paint a black line around the sleeve where it meets the wrist. I don't do it on all arms, just those that you would be able to see up the sleeve when viewing the figure.
Start simple and work your way up. Make simple or slight bends first until you get a feel for the material (plastic, styrene, resin, metal) and as you feel more comfortable then move up to broader animations.
Good luck.
Mike
"Imagination is the dye that colors our lives"
Marcus Aurellius
A good friend will come and bail you out of jail...but, a true friend will be sitting next to you saying, "Damn...that was fun!"