Sometimes it works and sometimes it doesn't. Kevin Costner turns in a wonderful comic performance. Roy is a little boy lost and Molly is his salvation. Some label Costner as wooden, but I think that's unfair. Prince of Thieves, but there are many actors who wouldn't be suited for certain roles.
Actors also have to be directed well.
Costner, however, has a gift of exuding an everyman quality and pulling us in to care about his character's plight. He does this with easy mannerisms and an interesting voice that causes you to hang on every word. It isn't necessary for him to get all intense and hyper in a performance because he knows how to use timing subtly. As for Rene Russo, she's one of the most beautiful women in the movies today, and this is her best role to date. Her character is smart and knows how to handle her men.