BERLIN (Reuters) - Director Steven Soderbergh said on Wednesday he so enjoyed making his psychological thriller “Unsane” on an iPhone, he would find it hard to go back to conventional filmmaking.

“Unsane”, which premieres at the Berlin film festival, was shot over just two weeks - way shorter than the months a movie usually takes.

It tells the story of Sawyer Valentini, who moves to a new city to escape her stalker David but finds herself admitted to a mental health institution where he works.

Sawyer, played by Claire Foy, is convinced she has been wrongly admitted to the facility but no one believes her so she is trapped there and subjected to torments from David, who gives her pills that make her lash out and imprisons her in a padded cell where he declares his love for her.

Soderbergh said the overall experience of making a film on an iPhone was good, although there were some drawbacks such as the phone being very sensitive to vibrations.

“I have to say the positives for me really were significant and it’s going to be tricky to go back to a more conventional way of shooting,” he said.

Not having to make a hole in a wall or secure a camera to the ceiling are big advantages, as is being able to go straight from watching a rehearsal to shooting, Soderbergh said.

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“The gap now between the idea and the execution of the idea is just shrinking and this means you get to try out more ideas so I wish I’d had this equipment when I was 15,” he said.

Joshua Leonard, who plays David, said filming on an iPhone enabled the actors to stay in the world of their characters and the film more than the conventional camera set-up would allow.

“There’s nothing more fun as an actor than just being in the thick of the creative process when you’re actually on set and not having to wait for the machine of filmmaking to catch up with the creative impulse,” he said.

Being used to people putting iPhones close to his face to take selfies helped too because “it really minimized any self-consciousness about the process of making a film”, he said.

“Unsane” is among around 400 films screening at the festival but it is not among the 19 movies vying for the main prize, the Golden Bear, which will be awarded on Saturday. The festival in the German capital runs until Feb. 25.