In keeping with your original premise (that of making formal photographs) having the mouth closed should be your ultimate aim.
I doubt if your garage was too warm, but it is an unfamiliar place, with unfamiliar people (you) and and unfamiliar action (posing on a stage).
Offering some suggestions to your humans, you can suggest that they go to touch the dog and reassure it that everything is o.k., then just as you are ready for the shot, you give a verbal command to the human, whereupon said human steps back out of the frame, then after the shot advances once again to the dog to keep it calm.
This would be a good time also to train your human to hold something of great interest to the dog in such a way as to "pose" the dog's head and eyes in the direction that would be ideal for a cherished heirloom of a photograph. I'd personally advise against an edible treat as you don't want lots of slobber showing in the photograph, or crumbs of treat on the dog's muzzle that you have to try to clone out later. A squeaky toy or a clicker would be my recommendation.
So in training your human, you would say: "Upon my command, step back across this tape I have placed on the floor, and hold the squeaky toy just "here" (where "here" is where you want the dog's attention) and give it a little squeak, but don't get your arm in my shot."
Oh, Man! Being an internet expert is FUN! And cheap! I should do this more often!