npc collider

I could be wrong when I say this, but I don't think the rigidbody counts as a collider itself, you use a rigidbody and a collider together.

Heres a great Unity Answer explaining:

http://answers.unity3d.com/questions/7671/guidelines-for-using-rigidbody-collider-characterc.html

EDIT: Personally I find you should only use mesh collider on objects or characters that have a complicated/uneven layout, like say a dumpster as a box collider would mean you would just hover over the opening if you tried to jump in it, therefore a mesh collider would be more adequent to let you stand in it.

For controllable main characters that are humanoids, its much more efficient to just use the character controller (which has a capsule collider included) rather than a mesh collider. Refer back to link if you want to use rigidbody though.

Mesh colliders are quite hard on the system to process as its like rendering the objects' mesh twice but one acts as a "shield" for the main skinned mesh. If you had every NPC and object with a mesh collider you would lower the framerate dramatically.