Speaking of Broadway and movies, casting for ‘Wicked’ — or at least, casting ideas — is brewing. I hear BRITNEY SPEARS has expressed interest in starring as the green witch. “She is obsessed with the musical,” a source says. “She listens to the CD all the time in her car. She bought 10 copies of the CD at a mall to give as gifts.” HILARY DUFF is another young songbird who may be in the running. She’s been talked about for the role of Glinda, the Good Witch. The show’s rep says that no one has been approached yet because the movie will not be made for quite some time.