You can see some work that I've done, both in studio and on location, on this company's site.
Some of the animal shots are from my stock; any image with humans was taken in my studio or on location, with instructions from an AD.
"Models" are local veterinarians and their clients, a hired actor/model, and some ad company personnel that were drafted. Everyone except the ad company folk were paid for their work. All pets are purely amateurs.
The ad agency received the raw files and all rights to them, and were billed accordingly.

Hey Ken,

It has been a while since you've been around - great to see you. This is pretty cool. How did this arrangement occur?

James

This job came about because an ad exec happened to be a client at the animal hospital that contains my studio. He was suprised that I did the work at that location (there are many dozen of my shots on the walls of the place.) A short discussion, and there was a contract.
I also got a job photographic auto parts and other mundane bits in much the same way.