Tim Burton insisted that the film be bloody, as he felt stage versions of the play which cut back on the bloodshed robbed it of its power. For him, "everything is so internal with Sweeney that [the blood] is like his emotional release. It's more about catharsis than it is a literal thing."
In some of the scenes where Sweeney opens his razor very quickly, it is a mechanical razor. There is a button Johnny Depp would press to have the razor open, which he hides with his finger. Depp personally requested the prop department design this razor for him, as he had difficulty handling the real razors.
Helena Bonham Carter was pregnant throughout filming with her and director Tim Burton's second child, and she insists (accurately) that the size of her breasts noticeably changes throughout the film, as filming was done out of sequence.
Before the role of Sweeney Todd, Johnny Depp had never publicly sung in his life. The first song he recorded for the movie was 'My Friends', which, in his opinion, was one of the hardest songs Sweeney sings in the movie.
Composer Stephen Sondheim, notoriously protective of his stage works, long resisted offers to film this story. When Tim Burton expressed interest, however, Sondheim relented when pleased with Burton's vision for the project and on the condition that the composer would maintain casting approval. Burton would only agree to direct with Johnny Depp in the lead, and though Sondheim feared Depp's vocals would be too "rock oriented," the composer approved the actor after a vocal audition. To approve the casting of Helena Bonham Carter, and to combat any rumor of nepotism (as Carter and Burton were romantically involved), the actress sent Sondheim no less than twelve audition tapes of her singing. Very impressed with her vocals, Sondheim immediately approved the actress. Also, in his recent book about his career as a songwriter, "Finishing the Hat" Sondheim states that this is the only adaptation of one of his works for the screen that he approves of.
The tradition for the stage productions of the musical has been to play Tobias as a grown man who is mentally challenged, not an actual little boy (as was the case in the movie). For example, Kenneth Jennings, the actor who played the role in the original Broadway production, was 31 on opening night; Neil Patrick Harris was 27 during the 2000 concert performances, and Manoel Felciano was 35 at the start of the 2005 Broadway revival.
When filming began, there was to be an inclusion of the spirits of Sweeney Todd's victims (including Anthony Head and Christopher Lee), who would sing "The Ballad of Sweeney Todd", its reprises and the Epilogue. These songs were recorded, but eventually cut since director Tim Burton felt that the songs were too theatrical for the film. Lee's appearance was completely cut from the film, but Head still has an uncredited one-line cameo (after the competition, Head's character asks Todd if he has a business of his own).
During its first month of production in London, Johnny Depp had to take a ten-day leave of absence when his daughter, Lily-Rose, was rushed to a hospital due to a severe illness (which was never disclosed by the media). To accommodate his absenteeism, director Tim Burton filmed scenes that didn't feature Depp's character.
Several songs from the Broadway musical were excised from the film. They are: "The Ballad of Sweeney Todd" (and its many reprises); "Ah, Miss"; "Johanna" (Judge Turpin's version); "Kiss Me"; "Parlour Songs"; "City on Fire"; and "Epilogue". All ensemble / choral singing was also eliminated, most notably from "God, That's Good", in which the title of the song is now never even sung. Many other songs remaining in the film have been shortened. The only songs to remain intact are "Epiphany"; "Pretty Women"; "Johanna" (Anthony's solo); "The Worst Pies in London"; "Wait" and "My Friends". "Johanna" (featuring Sweeney, Anthony, and the Beggar Woman) is originally a quartet featuring all three in addition to Johanna's vocals. The song "My Friends" is not sung in its entirety either, with one line being added and the last line was edited, from "At last, my right arm is complete again" to "At last, my arm is complete again."
The original Broadway production starring Angela Lansbury and Len Cariou opened at the Uris Theater on February 6, 1979, ran for 557 performances and won the 1979 Tony Awards for the Best Musical, Book and Score.
Imelda Staunton was considered for the role of Mrs. Lovett, but lost out to Helena Bonham Carter. She would go on to play Mrs. Lovett in the West End production of Sweeney Todd in 2012 alongside Michael Ball as Sweeney Todd.
The pies, or rather some of them, were actually edible. Some were made of fiber, others were made of rubber, etc. In fact, the pies in the beginning of the film were also edible and, according to Edward Sanders (Toby), even though they looked "gross", they tasted "okay".
The trivia items below may give away important plot points.
British playwright Christopher Bond wrote a 1973 play titled "Sweeney Todd" which was the first to give Todd a motive other than pure greed. Bond made Todd a wrongfully imprisoned barber who returned to London after fifteen years in Australia to find that the judge responsible for his imprisonment raped his young wife and caused her suicide. Bond's play was adapted in 1979 by Stephen Sondheim and Hugh Wheeler into the Broadway musical which the film is based.
In the stage musical, when Anthony comes to get Johanna from the insane asylum, Anthony is unable to kill the warden - so she takes the gun from Anthony and kills the warden herself. In the movie, this was changed to Johanna taking a more passive role in her escape and Anthony allowing the other inmates to kill the warden.
Todd is only seen completing two shaves, one on a man in the street for the competition against Signor Adolfo Pirelli and the other on the man who is with wife and children. Judge Turpin leaves before Todd can finish. All the other shaves end in Todd cutting their throats.
In the stage show Sweeney Todd kills Pirelli by slitting his throat but in the movie Sweeney Todd hits him over the head with a teapot exactly ten times. However, after being bludgeoned and thrown into the steamer trunk in one corner of the room, Pirelli's hand can be seen hanging out of the trunk and his fingers twitching. After a short, tense interchange with Toby, who has reappeared looking for his master, Todd bribes the boy to return to the pie shop for a "tot of gin", opens the trunk, pulls Pirelli's head back and finishes him off with one deep cut to Pirelli's throat.