Everyone has heard of a few things about facial expressions such as lip licking can be a sign that a dog is stressed. Whale eye is another one you see mentioned quite often. At the end of last year another paper was published looking at the differences in facial expressions in dogs when they were anticipating a reward after a signal that indicated a reward was coming and when they were getting frustrated due to not being able to get the reward that they expected they should have.
To help standardise the results they only used one breed, this was a good thing as different breeds won’t necesarily express emotions/frustrations in the same way. Does this mean that these expressions won’t be seen across breeds? Not at all! But it does allow the researchers to look at the different dogs within the breed and easily compare them. It also looks like they tried to remove people from the study so that the dogs wouldn’t offer any prelearnt behaviours, for example a lot of dogs will sit and expect attention, these could skew results.