Actress Julianne Moore has joined the cast of the Untitled Joseph Gordon-Levitt Project (once known as Don Jon's Addiction), which will serve as actor Joseph Gordon-Levitt's feature directorial debut.

We first reported on this project last month, when Scarlett Johansson signed on. Joseph Gordon-Levitt will also star as a Don Juan-type character, who goes on a journey to become a less selfish person. Julianne Moore will play a mentor to this lothario. It still isn't known what role Scarlett Johansson is playing.

Joseph Gordon-Levitt will direct from his own original screenplay, with Ram Bergman producing and Nicolas Chartier financing the project through his Voltage Pictures company.

Production is scheduled to begin next month. Julianne Moore was most recently seen on the HBO movie Game Change, where she portrayed Vice Presidential candidate Sarah Palin.