Just like with people, dogs may behave several different ways when they are fearful or anxious. The one we seem to recognize most easily as fear or anxiety is flight. Most people seeing the little guy in the picture to the left would say that they are looking at a scared dog. This is an example of the "flight" response. Frequently, their first response would be to help the dog feel more comfortable in their environment.
Do you know that dogs may also respond by fighting, freezing, or fawning? That dog that is barking, growling, and lunging at the end of their leash is in "fight" mode. People tend to label dogs whose reaction is forward when faced with something that scares them as "aggressive" when in reality, they are doing everything they can to get the scary thing to go away. Sure, they are behaving aggressively because they are afraid. Of course, this works! Who does not back away from a dog that is acting as if they want to eat you?! If you are dealing with a dog that demonstrates fighting behavior, you are likely have a fearful or anxious dog that needs help understanding that they are safe in different situations.
While freezing is fairly self explanatory (your dog becomes immobile, even unable to move), the "fawn" response is less well known. You dog may try to appease or please the perceived threat in order to avoid harm. Sometimes this can look like a dog that is overly friendly. Sometimes it looks like a dog becoming "small" and unnoticeable.
While fidgeting is not commonly identified as a "fear response", it can be an indication that a dog is uncomfortable. They are not able to settle or may fuss with their toys (or other objects) or even their people.
If your dog is displaying any of these behaviors, work on building their confidence and slowly introducing them to the situations that make them uncomfortable. Always be sure to teach them alternate appropriate behaviors outside of the scary situations BEFORE you expose them to the scary stuff. If you dog is fearful or anxious, they acting on instinct and unable to think. If they can't think, they can't learn. Don't try to teach in the situations that cause your dog to have a fear reaction. They are in survival mode, not learning mode. Get them out of the situation and, if needed, contact a professional to help you.