Paris Hilton's mother Kathy stars as Tom Cruise's on-screen wife in new movie Tropic Thunder.

The 49-year-old beauty - who is mother to socialites Paris and Nicky as well as sons Barron and Conrad - kept her appearance in the Ben Stiller movie a secret from her family, but "raved" about her new co-star when her cameo was finally revealed.

A source said: "She filmed last winter and didn't even tell her kids because she wanted to surprise them. She raved about Tom, saying he was so warm and friendly and made her very comfortable."

This is not Kathy's first acting role. She has also starred in numerous episodes of hit US TV show The Young and the Restless.

Paris is also hoping her acting career will soon take off, after her appearances in horror movie House of Wax and comedy The Hottie and the Nottie were panned by critics worldwide.

Tropic Thunder, which sees white actor Robert Downey Jr. play an African-American man, will be released later this year.