Williams has been offered the role of US navy officer Nelly Forbush, who nurses injured soldiers and sailors during World War II, in the big screen adaptation of the hit stage musical, reported Contactmusic.
Timberlake is also set to be offered a role in the film as young officer Lieutenant Cable.
However, the movie - which will be directed by Michael Mayer - is still in its early stages and pre-production hasn't started yet.
Williams, who showed off her musical talents when she played Marilyn Monroe in 'My Week With Marilyn', has previously spoken of her desire to star in a musical.
"I think maybe that's why Marilyn was so especially talented at it. Her singing and dancing are unparallelled and her musical numbers are just breathtaking," she had said.