texturing for photorealism question

ok lets say I have a grey alien model, and my end result is for photorealism as I am looking to do animation with him and insert in live footage.

How much of the end result photorealism has to do with the texturing? Which parts specifically?

I've done some 3d work but ain't no expert or I would not be asking.

Cheers,

Depends on a lot of things. But if you're going for a photorealistic look the textures will make or break it. The model can be low poly (and therefore not to realistic, if its a human, or something with a lot of detail) and the texture (Diffuse, normal map, etc) if made to look realistic will really be most important in creating a realistic look. (Lighting can also be very important. Good lighting can make a mediocre model look great/real).