I only know this because of Bill Simmons, and it has been a while for that guy, but doesn't Costner also do like a bad Boston accent, a bad Scottish (maybe Irish) and a bad southern, or something? Like he has done bad accents of all the great bad accent accents' as well as his ability to pull of bad accents in a wide variety of novel ways.
(the only two costner movies i have seen are field of dreams and 13 days. i saw field of dreams when i was like 10, and i liked baseball so i liked it. i saw 13 days for a class in college, and failed the class because after that i did not go back. it was because we had to watch another movie and i absolutely hated watching movies in college. we were paying good money to learn, not watch some bullshit movie. also, i didn't like the teacher, maybe because of the movie thing, i don't think so but can't say for sure. anyways, the other movie was dr strangelove, which i have seen since then and it is one of my favorites. probably should have just gone, but probably wouldn't have enjoyed it in that situation anyways? anyhow, i actually went back one more time for the first test, and it was three essay questions two about movies, i had only scene one and hated it/ couldn't really watch it so i didn't do to well. this was a poli sci 4000 level course. oh yeah, the other point, i have absolutely no idea what i am talking about with the accent thing.)