Question: How did Kevin Costner and Whitney Houston's relationship evolve after The Bodyguard?
It didn’t.
What people seem to forget is the same year that The Bodyguard was released. Whitney Houston married the love of her life and the father of her only child, Bobby Brown.
Although Brown had drug problems and was alleged to have been unfaithful to Houston during the course of their marriage, she stood by him when he went to jail and she even changed aspects of her career to accommodate the fact that his own career had fallen by the wayside due to his abuse of drugs and alcohol. While she apparently had minor relationships following their 2007 divorce, she never again married nor was seriously involved for the remaining five years of her life.
Whitney Houston and Kevin Costner starred in a single film that was a surprise blockbuster hit. Due to Costner decisions to make different films, and Houston’s personal problems derailing her career, they never acted together again in a project. However people who are romantics have ascribed to them a mysterious relationship which was apparently so secretive that even they didn’t know anything about it.
Both had their own lives and following The Bodyguard those only occasionally intersected.