There’s post processing methods…
Is it just characters, not environments? If so, in your 3d creation package, the old school trick is to duplicate the character mesh, inflate it a bit so it’s a teeny bit bigger than the original. On this duplicate, invert the normals and give it a new materials. Update the skins on the duplicate and export both as a single asset. In UDK, make a material to assign to the duplicate and keep it as a simple color. If you know how, you can also make it draw after solid geometry, but it’s not necessary to achieve the effect.