There's an interesting bit of trivia related to that.
I loved "Robin Hood: Prince of Thieves"(still do, even though critics may disagree) but I always wondered why Costner never bothered to even attempt an English accent for the role of Robin.
Then I read that there was another Robin Hood movie in production at the time. Both companies wanted to be the first to get their film to the screen. The plan was for Costner to dub in his lines at a later date, speaking in more of a British accent, but they were so rushed that they skipped the dubbing and let his dialogue go as it was. They won the race, too.
Interesting tidbit. Last time I saw Costner recently was video of him speaking at Whitney Houston’s funeral. Oh, and AARP magazine did an article on him. I’m feeling old just thinking that AARP was featuring Costner.