One of the secrets behind Shadowgun's impressive graphics!.
Instead of rendering your normal maps at runtime or trying to laboriously paint fine detail onto your textures, try baking normal mapped lighting in the editor! It's like lightmapping, but for high-frequency light (fine details).
Also, if you ever wanted to use world or object space normal maps, or use the texture space lighting technique used to render subsurface scattering on faces in GPU Gems 3, this script would be a great starting point.
