For a number of years he portrayed people as angels. I wrote about that here.
Thayer seems to have been what is now called "bipolar," suffering extreme mood swings. By the time of the Great War he had given camouflage a good deal of thought and pushed his ideas on various military organizations.
"Thayer reveals that his method was to work on a new painting for only three days. If he worked longer on it, he said, he would either accomplish nothing or would ruin it. So on the fourth day, he would instead take a break, getting as far from the work as possible, but meanwhile instruct each student to make an exact copy of that three-day painting. Then, when he did return to his studio, he would (in his words) 'pounce on a copy and give it a three-day shove again'. As a result, he would end up with alternate versions of the same painting, in substantially different finished states."
Below are some examples of his non-angelic works.
This is one of the earliest Thayer paintings I found on the Internet. He was about 35 years old when he made it. An interesting pose, and the subjects are convincingly rendered.
This was made about the time he was beginning his angels series that lasted until around 1904.
He made at least two paintings with a Virgin theme.
This is another one.
By the early 1890s his technique became less "finished" in many cases.
One of his models, probably a teenager when this was painted.
Same subject a year later. She seems considerably more mature.
The first of at least two studies of a woman named Alma Wollerman. For some reason Thayer seems to be having trouble depicting her eyes. (This was too early to have been influenced by modernists such as Picasso, and I doubt Thayer was very interested in Modernism as practiced in France.)
Another Wollerman study. Again, the eyes are not convincing.
Portrait (study?) of his daughter Gladys -- quite roughly done, and again the eyes don't seem quite correct.