i thought normal mapping and bump mapping were the same?? 
normals take into consideration the eye of the viewer, the position of the light hitting the face being viewed, and the orientation of the face being viewed in relation to an origin of coordinates.
if you apply a normal map to a character's face texture, it will darken when a light hits it, giving the illusion of added volume.
if you move the light around, the normals will compensate with added brightening or darkening, but it most likely will never be fully dark or fully bright.
that saves you a lot of calculations if you consider regathering or caustics.
bumps, on the other hand, "push" the geometry always away from the normal of the face being viewed.... so if you apply a bump map to the same character texture, say, with a bump to depict the nose or the cheekbones, as you move around the face you'll notice excessive deformations around those bumps.
the area being viewed stays the same, and you move around, but the shader is not considering that into the rendering.
... think of it as Indiana Jones' "Leap of Faith" illusion.... as long as you watch it from the right end, it appears to have volume, but as you deviate from that angle, the volume doesn't follow your 3D shift. (or rather.... you brain knows something just isn't right)
simply put: you can't see volume if you don't consider light, reflection, and shade.... bumps are a fixed gradient of light and shade, and normals are orientation-based gradient of light and shade.
