The Impossible star reveals she was impressed by the Oscar winner's portrayal of a rich socialite plunged into poverty and homelessness, and poured her heart into a note for Blanchett.
Watts says, "Cate Blanchett's performance was out of this world, she was just fantastic; I wrote a fan letter to her.
"Just (telling her) how brilliant she is, as always, but particularly this role... I laughed, I cried, I cringed, I did all things, and so did she. She was just brilliant."
Watts wasn't the only member of Hollywood's elite in awe of Blanchett's talent - she was named Best Actress at Sunday's (02Mar14) Oscars ceremony, earning the Australian her second Academy Award.