The celebrity photographer told American Vogue that playing music by French singer Dalida — the same tracks used while he photographed Diana, William’s late mother — was the secret to the royal couple’s relaxed poses.

"The contrast of the contemporary informal music playing in the beautiful rooms with so much history caused a lot of laughter," he was quoted as saying.

William and Middleton chose two images for release after the photo shoot at St. James’s Palace last month, shortly after they announced their engagement. The couple, set to wed April 29, looked relaxed and happy in the images.

Middleton was an "easy sitter" because of her natural grace, Testino said, adding that she was humorous but also appeared to have developed "a perfect sense of royal responsibility."

The Peruvian fashion photographer has worked with William many times in the past, and is a royal family favorite who captured some of the most iconic images of Diana. A series of glamorous black-and-white images he took of her were published in Vanity Fair in 1997, shortly before her death in a Paris car crash.