The obvious answer is... One that wants to be left alone.  But my guess is, that's not the answer you're looking for.  We really need more information to try and answer this one for you, because many snakes will flatten their heads when they feel threatened to try and look more like a viper. 

 

If you're in the southwest, and it has rattler coloring.. Browns and tans.. My guess would be that it's a gopher snake.