The Don't Worry Darling director lit up the red carpet during the San Sebastian International Film Festival in Spain.

 Where the thriller premiered at the Victoria Eugenia Theatre on Sept. 17.

For the special occasion, Olivia glimmered in a mermaid green halter dress by Valentino. 

The design, which debuted on the 2022 couture collection, molded her body with its curve-hugging material.

However, it was when she turned around that the gown, well, truly shined.

As it featured a plunging open-back that was cut so deeply her side-boob peeked out.

One downside to the eye-catching number was that the floor-length train was too long for the 38-year-old.

 Who almost took a tumble while she made her way through the red carpet. 

 Photos captured Olivia trying to balance herself as the dress snagged onto her sequins pumps.