I can bet you've heard it said that you should withhold the food treat from a fearful dog as otherwise you are just rewarding the fearful state.
This old chestnut is so engrained in dog training mythology that it can be hard to convince some people that this is not the case.
Let's be clear - You cannot reinforce an emotion, only a behaviour, and if you are wanting to change a behaviour the first thing to do is change the emotion underlying the behaviour.
Let's dig deeper.
Firstly there are two types of learning happening every time an animal (human or otherwise) makes contact with his/her environment - firstly there is Pavlovian learning (respondent) and this type of learning occurs whether you like it or not. An example would be that if every time you went to the dentist something painful and scary happens you very quickly learn that you are not fond of the dentist. Dentist = Pain. Simply the clinical smell in the reception might make your palms sweat. It might take a lot for the dentist to change this association but, if after several more visits where nothing painful happens, and instead the dentist rewards you with free dental prophylaxis, (and imagine if he gave you money for coming!) you might begin to change your opinion of the dentist. You might drive up and find yourself anticipating a reward. You can see that it is harder to change the associations if there have been lots of unpleasantness to begin with. Maybe one time the local anaesthetic failed. This is why starting on the right foot when it comes to vet visits is so important - you may not have enough vet visits in your dog's life time to overcome a really bad first experience.
The second type of learning is operant learning and is the type of learning an animal experiences when it "operates" on his/her environment. The animal does something and a consequence occurs. In this type of learning an animal (human or otherwise) learns that their behaviour has a consequence and if the consequence is pleasurable (to the animal) then they are more likely to repeat the behaviour (reinforcement) and if the consequence is unpleasant ( to the animal) then they are more likely to not repeat the behaviour (punishment) in the future. This kind of learning is what we use when we cue a dog to do a behaviour and then reward it with a treat following that behaviour, thereby increasing the likelihood of the behaviour occurring more in the future.
Now, so let's think about the fearful dog in the consult room of a vet clinic. Is it correct to feed this fearful dog even if he/she is growling at the vet? YES. Because the first thing to do in this instance is to attempt to change the emotion behind the growling. Here we are working to change the association the dog has with the vet clinic from unpleasant to pleasant. The growling is telling you the dog is fearful and wants you to go away and the first task is to attempt to make the dog feel less scared. If supplying food encourages the dog to feel more comfortable then YES food should be used. This is not making fear more likely in the future. Should the caregiver reassure the dog - Again YES the caregiver can reassure the dog - this should not be in scared, worried tone however but more about the caregiver demonstrating that they are calm and unworried. This modelling of calm trusting behaviour could be useful for the dog that has come to look for guidance from his/her caregiver.
Being comforted by someone when you are fearful does not make you more fearful in the present or the future. For example, imagined being comforted by an airline steward that the aeroplane is safe and the turbulence being experienced is completely normal. You might even feel better. Whereas being reassured by a jittery passenger next to you who is screeching in a panicked tone "it'll be alright, I promise" could actually make you feel worse. Being comforted by someone who is also scared and worried may not be helpful. So when a caregiver is attempting to ease a dog's worry it is best if they can demonstrate that they are calm. They need to do this with tone of voice and movement of body. They may need to practice this.
Commonly a scared dog is too scared to eat ( a physiological response telling the animal that this is not time to chomp down) and so food cannot be used to change the emotion in this setting. If this is the case the intensity and perhaps the distance to the triggering person/event/place needs to be changed to allow the food to be taken. Using food ( a natural motivator) is the most sensible and humane way to change emotion and consequently behaviour. Some dogs are more food driven than others and some dogs will still eat even though they are terrified so reading the whole dog is required.
So if we look at what some people advise - which is to tell the dog off for displays of aggression we can now see how this is unhelpful. Telling a dog off for a fearful display does nothing to alleviate the dog's concerns and may increase a sense of unpleasantness around the context in which the telling off is occurring and impart a distrust of the caregiver too. Temporarily stopping a display of growling is often unhelpful and can be seen to be not effective as in fact the situations in which growling has occurred has increased or evolved to more and more scenarios and may have even morphed into more serious displays including bites (which now seem to have less warning).
If you are struggling to change your companion's behaviour first look at what is motivating your animal to behave the way he/she is. No doubt there is a good reason ( in the mind of the animal) and your job is to change the emotion so the behaviour no longer needs to be performed to keep him/her safe. Animal Sense can help you construct a humane Behaviour Change plan.