The actress returns to the role of Belle de Jour/Hannah Baxter next Tuesday in the fourth and last series of the ITV2 programme.
Asked if the show had moved on, Piper told Heat: "I think it's more ambitious now. Reality and fantasy have merged a bit more, plus she's totally lost it. She has no moral compass now.
"And the sexy side of things is funnier. It's still quite shocking, but we play it for laughs now a bit more. It's fun. Less soft porn, more weird."
She continued: "One of her clients likes to dress up as a baby and be mothered. And these people do exist, by the way. They like to be cradled while wearing a nappy.
"When I first read the script I thought, 'We can't do this; it feels incredibly wrong'. And also, how do you do it so it's not just disgusting? But we found a way to make it work, and in the end it's very funny, I hope."
Asked if she actually stripped naked for the nude scenes in the series, Piper said: "No. There are scenes of bare breasts, but I'm afraid they're not mine."
> Billie Piper: 'I can't do intense grinding'