I just did this with some NECA Predators (elbow and knee could not move at all before). I used boiling water, but you could probably use hot water just before it fully boils. Dip the joint that you want to loosen (you can wear gloves or just be careful) and stir it around for a few seconds. Take it out and see if the plastic is soft enough. Figure will be hot so be careful. Try to slowly and carefully move the joints. Hold the figure as close to the joint as possible while moving it because the plastic might be "too soft" and you could tear it if you are holding it too far away from the stuck joint. You might also want to "push in" on the joint while you're moving it to compress it a little to give it some more rigidity. After you move it several times, let it cool down (you could use cold air or water).
You could also try using a dropper and drop hot water into the joint if you don't want to wet the whole figure (never tried this way though, might be too slow).