For his performance as Jett Rink in Giant (’56), James Dean dug himself into a very deep mannerist hole. His Texas cracker accent was thicker than bean dip, and made his voice sound even more nasally and high-pitched than usual. And it was totally at war with consonants. The idea, apparently, was to un-enunciate as much as possible, speaking almost entirely with mood sounds and surly slurrings. If he’d gone all the way Dean would have avoided consonants altogether. Try to say “mah well came in, Bick” without the first letter in each word. I just did and it sounds something like “ahh ehhl ‘ame ihn, Ick.” That‘s what Dean was shooting for.**
The Wiki page says that “after Dean’s death late in production, Nick Adams overdubbed some of his lines, which were nearly inaudible, as Rink’s voice.”
Significant anecdote: My ex-wife and I named my eldest son Jett because of Dean’s performance. The actual suggestion came from publicist Bruce Feldman.