The Scottish actress told The Daily Record that she was instructed to modify her speaking voice for the benefit of American viewers.
"The director said I needed to moderate my accent a bit to make it lighter and therefore more understandable," she explained. "I thought my voice was quite soft and light anyway. I certainly don't have a broad Aberdeenshire accent, but that was the message I got."
However, Manson confirmed that filming the eight-part series had been "great fun".
"I love all the action stuff and there's plenty for [my character] Fleur to get her teeth into," she said. "I like the adrenaline rush of speed and danger. I like to feel frightened."
She added that Fleur will act in a "truly shocking" manner following the death of a friend in the show's first episode.
"She realises she has to stand up for herself," said Manson. "What she does... in response to what happened to her friend, is truly shocking."
Outcasts begins this Monday at 9pm on BBC One.