Kevin Costner, fresh off his "Dances With Wolves" Oscar sweep, isn't taking much time off to savor his success. Costner stars in "Robin Hood: Prince of Thieves," set for a June release, and is in Dallas filming "JFK." In that Oliver Stone-directed movie, Costner plays a district attorney who investigated President Kennedy's assassination. In the fall, Costner will play opposite singer Whitney Houston in a film called "Bodyguard." And next year he's been lined up to star in "The Mick," about the life of Irish patriot Michael Collins.