Rosamund Pike has confessed it felt "inappropriate" when director David Fincher left her and Neil Patrick Harris alone to rehearse their sex scenes for Gone Girl.

The actress stars as Amy in the big screen adaptation of Gillian Flynn's hit thriller, with Ben Affleck playing her husband and Neil cast as her strange ex-boyfriend.

Rosamund told US Glamour magazine she tried to lighten the mood in the sex scenes by joking around, as they did have to go to a "dark place".

The 35-year-old British actress revealed: "Ben and I would have these scenes that were very giggly and sort of romantic, and then we'd go to this very dark place..."

She said of working with her other co-star and how he broke the tension: "Neil was incredible at that. We were in hysterics in the editing room.

"It was very funny doing [rehearsals for our sex] scene. David left Neil and me alone on that set for like two hours to make sure we could do it. But when it's just two of you, basically kind of f***ing on a bed, it feels so inappropriate."

Rosamund admitted: "There were definitely moments when I felt very uneasy with how much power Amy had.

"And the interesting thing is, my reaction was to laugh, to get sort of uncontrollable giggles. That was the most curious part of playing her: At the point where she's at her most powerful, it unnerved something in me."