The actress has really enjoyed shooting Sam Raimi's forthcoming fantasy film as it's helping her six-year-old daughter understand why the fictional Oz is such a wonderful place.
She tells 24 Frames, "It's been such an amazing move for us as a family. It's a really happy place to go to work and a happy place for her to come visit."
And Williams is glad she can take her daughter to work after insisting she stays away from more serious locations.
She adds, "The (My Week With) Marilyn sets and the Blue Valentine sets - they aren't really ideal places for children. She can still come visit, but she can't really get comfortable and hang out for a while. This really integrates my life and my work in a great way."
And Williams admits she was well prepared to play Glinda the Good Witch - because Matilda loves Judy Garland's classic movie musical The Wizard Of Oz, where the good-natured character was introduced to the cinema world.
She adds, "I've already watched The Wizard Of Oz a lot, because it's one of my daughter's favourites."
Matilda is Williams' daughter with late actor Heath Ledger.