Just a quick thought on testing the water.
Ok, it sounds obvious, but to test the water, why not try a cosey night in. I remember introducing a girlfriend into BDSM by showing her the famous film “Exit to Eden” It at least allows you to broach the subject and gauge response without a full on “questioning”
Also I think perhaps a better film to show a partner is “The secretery” which is an excellent film to introduce the subject. It’s good. I bought it. You can even get it cheap and discreet on Amazon
When I first saw this film, at the cinema, it was quite interesting because looking at peoples reaction at the cinema. Some, who understood the gift of submission thought it excellent and a riviting film to watch. Then there was the general public who didn’t understand. They laughed, out of ridiculing what they do not understand. However I really recommend you check it out! Good film, and planned right, a good intro to BDSM for a potential vanilla partner.