The former Doctor Who star explained that her character Belle De Jour's mental breakdown during the fourth run of the ITV2 drama will be reflected in the show's elaborate wardrobe and make-up.
Piper told Buzz Magazine: "We've taken costume and hair even further this year... Belle has got to that point where she is borderline insane, and we reveal that through her clothing and make-up.
"She's starting to lose herself in the world of prostitution. The show is about to show that through what she wears. The costumes are insane this year."
Commenting on the decision to end the scandalous programme, Piper clarified: "Arguably I’d play Belle forever if there was less sex and a three-day week... Oh, and a body-double on standby the entire time, so I could feast on good food and not worry about turning into a beached whale."
Secret Diary Of A Call Girl continues Tuesday at 10pm on ITV2
> Billie Piper teases 'weird' sex scenes