Blake Lively's ring for 'A Simple Favour' was inspired by the late Princess Diana.

The 31-year-old actress' alter ego Emily Nelson wears an eye-catching diamond-and-sapphire band that used to belong to her mother-in-law in the thriller and it was a ''take'' on that given to the late royal by Prince Charles when they got engaged - and which was given to Duchess Catherine by their son Prince William on their engagement - because director Paul Feig wanted a traditional piece that jarred with the fashion publicist's usual style.

Costume designer, Renée Ehrlich Kalfus explained to People magazine: ''It's a take on Princess Diana's ring...I wanted to do something vintage, but the director thought it should be more on the nose.''

The ring serves as an important plot point in the new drama, which follows Stephanie (Anna Kendrick), a mommy vlogger who seeks to uncover the truth behind her best friend Emily's sudden disappearance.

When it came to Emily's wardrobe, Renee drew inspiration from the movie's own director in order to kit out Blake's character.

She explained: ''Paul goes to work every day in a Savile Row suit. He feels he can get anyone to answer anything while wearing a suit.''

And as well as her diamond-and-sapphire ring, the businesswoman has a lot of unusual jewellery from men's collections to wear with her Ralph Lauren suits.

Renee explained: ''It gave her more armour, more stuff to keep people away.''

Renee relied on Blake's own contacts when it came to sourcing spectacular footwear for the character.

She said: ''Because Blake has a relationship with Louboutin, we got boxes and boxes of boots, loafers, and heels.

''Blake had to run in them and she was like, 'It's fine, I can do this.' ''