From June 1939:

Clark Gable, even, is taking his role of Rhett Butler very, very seriously. More seriously than any he’s every played before, his pals tell us. But he’s not going to try to talk with a Southern accent.

So different from Vivien Liegh, the English gal who got the Scarlett O’Hara role. Vivien IS developing a Southern drawl for the film. Even to the extent of snooting her English pals, these nights. She turns down their dinner and party invitatons with this classic–”So soddy, my deahs–but I just cahn’t afford to be exposed to your broad A’s. I’ve got to talk Southern, honey!”