She then played Mrs. Lovett in Sweeney Todd , which was also released in 2007 . She then played Mrs. Lovett , Sweeney Todd 's ( Johnny Depp ) amorous accomplice in the film adaptation of Stephen Sondheim 's Broadway musical , Sweeney Todd , the Demon Barber of Fleet Street . The film was released on 21 December 2007 in the US and 25 January 2008 in the UK .