According to The Mirror, producers altered Miller's appearance so that nude scenes in the 1960s-based movie would look more authentic.
"The film is set in the swinging '60s when fashion was wild and body hair even wilder," said a source.
"Unfortunately, Brazilians weren't common in the '60s and Sienna's part involved one or two nude scenes - meaning that her grooming habits were on full display."
They added: "Sienna's private parts were digitally enhanced, giving her a rather unruly, loud and proud bush.
"All the cast had a good giggle about it and stoical Sienna happily played along."