Rebecca Ferguson's voice was dubbed by Loren Allred. Ferguson had studied music and admitted that she can carry a tune but since Jenny Lind, her character, is considered the best singer in the world, dubbing her voice would be in service of the movie. However, in order to get into the role, Ferguson insisted on singing the song in front of the extras while filming.
According to Hugh Jackman, the film's nine-year development process from conception to completion was, in part, due to studios' unwillingness to take a risk on an original musical. What finally sold the deal at 20th Century Fox was the future Oscar-nominated song "This is Me," which Benj Pasek and Justin Paul wrote during the two-hour flight to the studio meeting where the film was greenlit.
Many of the costumes used in the ensemble circus scenes at the beginning and end of the film were borrowed from Feld Entertainment, the current owners of Ringling Brothers and Barnum & Bailey Circus, and were used in productions of "The Greatest Show on Earth."