The actress will appear in new ITV drama Leaving about a relationship between a couple with a 20-year age gap.
She explained that Homeland star Lewis thought she was "mad" to turn down roles because of nude scenes.
"He said to me, 'I think you're mad to always say, no, no, no. It's bonkers'. So I listened to him and I took the role. And I'm really happy that I did," she is quoted as saying in The Sun.
"It was totally freeing. In fact, I doubt I shall ever be clothed on screen again!"
The Harry Potter actress added that she feels now completely different about appearing naked on screen.
"At 21, I thought that kind of thing was exploitation. But now I'm in my 40s, I think it's art."
The three-part series Leaving will air on ITV1 in September.