The usual background to a scared / nervous dog is a series of events which put it into this state. For instance a common incident would be taking your dog for a walk and it sees another dog and either barks at it or stops and turns into a lead weight which you can’t move, often curling into a ball with its tail between its legs.

Common signs of a scared / nervous dog is barking, erratic behaviour, extreme wagging of the tail while moving around another dog, tail between the legs, growling, turning into a lead weight and not moving. Extreme cases your dog may even turn into a catatonic state, where its muscles get so ridged that it is virtually impossible to move it.