The actress was desperate to work with the Danish director, although she worried she didn't fully understand his complex script, about a couple coping with the loss of their only child.
Gainsbourg's fears intensified after von Trier took her aside onset and confessed he was struggling with debilitating anxiety he sometimes felt unable to control.
She recalls, "I hadn't met him before so I didn't know what kind of man he was, but he did explain in a very simple way that he had had a depression, that he was under heavy medication, and just trying to deal with his panic attacks that he had all the time.
"That was scary because he came up after the first week, to me and to Willem (Dafoe, co-star), and explained that he didn't know if he could finish the film, that he didn't know how to cope with his attacks, that I would have to excuse him if he ran away, and I was terrified at the thought that he would leave me - leave me on my own to make the film.
And while critics booed von Trier's violent and graphic movie at the Cannes Film Festival in France in May (09), Gainsbourg was still lauded for her performance, taking home the best actress prize at the prestigious event.