After an extended break, Piper decided to end her pop career in 2003 and return to her original ambition, acting. She took acting lessons while living in London. Piper gained very positive reviews for these appearances, critics seemingly feeling that she was a far better actress than she was a singer. In 2004, Piper appeared in the films The Calcium Kid, as the romantic interest of Orlando Bloom's character, and Things to do Before You're Thirty. Shortly before starting work on Doctor Who, she filmed a starring role in the horror movie Spirit Trap alongside Russian pop star Alsou, released in the summer of 2005 to generally poor reviews.

In November 2005, Piper starred as Hero in a BBC adaptation of Much Ado About Nothing, updated for the modern day in a similar manner to the Canterbury Tales series in which she featured, with Hero now being a weather presenter in a television station. Piper has completed work on two stand-alone television productions. In the first, a BBC adaptation of Philip Pullman's historical novel The Ruby in the Smoke broadcast in December 2006, Piper played protagonist Sally Lockhart, a Victorian orphan. The BBC plans to film all four of Pullman's Sally Lockhart novels, with Piper continuing in the role in The Shadow in the North which was shown in December 2007. In 2007 she appeared as the main character, Fanny Price, in an adaptation of Jane Austen's novel Mansfield Park, screened on ITV1. This was her first acting role on television for a broadcaster other than the BBC.

Piper has provided voiceovers for various television commercials, including one for Comfort Fabric Softener airing in June 2007. Piper also stars in Secret Diary of a Call Girl, an ITV2 adaptation of Belle de Jour's The Intimate Adventures of a London Call Girl, a memoir detailing the life of a high-class prostitute. The series, which aired from 27 September 2007, saw Piper in several semi-nude scenes, including a lesbian romp and one scene featuring her saddling a client and riding him like a horse. A second series, with Piper in the starring role, started filming in May 2008, during which two body doubles were hired in order to hide Billie's pregnancy.

• "Secret Diary of a Call Girl" .... Belle / ... (16 episodes, 2007-2008)
       - Episode #2.7 (2008) TV episode .... Belle / Hannah
       - Episode #2.8 (2008) TV episode .... Belle / Hannah
       - Episode #2.6 (2008) TV episode .... Belle / Hannah
       - Episode #2.5 (2008) TV episode .... Belle / Hannah
       - Episode #2.4 (2008) TV episode .... Belle / Hannah
        (11 more)
• "Doctor Who" .... Rose Tyler (33 episodes, 2005-2008)
       - Journey's End (2008) TV episode .... Rose Tyler
       - The Stolen Earth (2008) TV episode .... Rose Tyler
       - Turn Left (2008) TV episode .... Rose Tyler
       - Midnight (2008) TV episode .... Rose Tyler
       - The Poison Sky (2008) TV episode .... Rose Tyler
        (28 more)
• The Shadow in the North (2007) (TV) .... Sally Lockhart
• Mansfield Park (2007) (TV) .... Fanny Price
• The Ruby in the Smoke (2006) (TV) .... Sally Lockhart
  ... aka Sally Lockhart Mysteries: Ruby in the Smoke (USA: DVD box title)
• "ShakespeaRe-Told" .... Hero (1 episode, 2005)
       - Much Ado About Nothing (2005) TV episode .... Hero
• Spirit Trap (2005) .... Jenny
• Things to Do Before You're 30 (2004) .... Vicky
• The Calcium Kid (2004) .... Angel
• Bella and the Boys (2004) (TV) .... Bella
• "Canterbury Tales" .... Alison Crosby (1 episode, 2003)
       - The Miller's Tale (2003) TV episode .... Alison Crosby
  
• "Music Chronicles" (1 episode)
       - The Advantage (????) TV episode
• Evita (1996) (uncredited) .... Undetermined Role
• The Leading Man (1996) (uncredited) .... Bit Part