Scarlett Johansson found it ''challenging'' to play the central character in new film 'Lucy'.

The flick - which also stars Hollywood legend Morgan Freeman - follows the story of Scarlett's character, whose brain becomes extraordinarily powerful after she becomes a drugs mule for her new boyfriend.

The actress admits she found filming the much-hyped flick to be an ''intense'' experience because there were ''no guidelines'' for the role, telling BBC Radio 1: ''We shot part of the film in Taipei [Taiwan], which was very exciting.

''Even making it everyday, sort of, you know, brought on its own unique challenges because the character was losing the sense of her own humanity as she's gaining all of these superhuman abilities.

''So there was kind of no guidelines for that. I never knew what I was going to be stepping into each day. The whole shoot had a level of surrealism to it, I suppose.''

She added: ''It was sort of an abstract experience. I don't know, I enjoyed it.''

Scarlett, who is rumoured to be expecting her first child with fiancée Romain Dauriac, also revealed she has just finished shooting her scenes for the 'Avengers' sequel.

'Lucy' opens in UK cinemas on August 22.