Stockholm director Robert Budreau was relieved he didn't have to shoot a full-on sex scene in his new movie, because it was such a drama to film Born to Be Blue's raciest moment.

In Stockholm, Ethan Hawke's bank robber character gets it on with a clerk, played by Noomi Rapace, fully clothed in front of her fellow hostages, and the moviemaker insists it was a much easier shoot than the last time he and Hawke shot a sex scene.

"It was one of the most important yet tricky aspects of the story that after a couple of days together in this vault that they (Rapace and Hawke's characters) would fall for each other and literally have sex in the vault with other people in there," the moviemaker explains. "But that is what happened.

"We just tried to approach it very naturalistically and those are my favourite moments. It came out of something real. There was a lot of pressure to get that right because it's the defining moment of the film.

"But it wasn't a full-on sex scene that you had to be precise about nudity. That becomes a bit more technically challenging because you have to be more particular, which is a drag because your predefining the scene.

"In this (Stockholm), their clothes stay on, so they basically can do whatever. In Born To Be Blue, the jazz movie I did with Ethan, there's kind of a sex scene with him and Carmen Ejogo, so that was a bit more choreographed.

"His pants come down and he goes between her legs and there's certain angles we have to shoot that we can't show, so that certainly became an example of a bit more choreography. It wasn't hardcore but it was certainly taking it to the next level."

Stockholm, which is based on a real-life bank robbery which coined the phrase 'Stockholm Syndrome' - linked to captives who fall for their kidnappers, was a big hit at the Tribeca Film Festival.