This can be seen in the relative ease with which actors transition from one to the other. Think of Vivien Leigh in Gone with the Wind. It also works the other way 'round. A striking example would be John Hillerman, a native of Dennison Texas who played the upright English major-domo Higgins on Magnum PI. Hillerman sometimes played Higgins' Texan cousin Jim Bob during the series. Jim Bob's Texas drawl is in fact Hillerman's natural speach, while Higgins' pseudo-Oxonian is a put-on.
I would include in the overall Midwest definition those parts of Montana, Wyoming, and Colorado east of the Front Range, as well as western Pennsylvania and northern West Virginia due to the similarities in economy, population, and language to their Midwestern neighbors. I would exclude Oklahoma due to the predominantly Southern influence on that state.
Certain Midwestern distinctives, like the hard "r", may reflect the Germanic roots of much of the region's population. Regarding Gone With the Wind, it is worthy to note that Clark Gable, a native of Cadiz, Ohio, and partially of German ancestry, did not even attempt to sound Southern. During the Golden Age of movies, a large number of actors from mid-America did not try to sound Southern, even when their roles called for a drawl: John Wayne (Iowa and California); James Stewart (western Pennsylvania); Gary Cooper (Montana). OTOH, in the recent movie Cold Mountain, only one of the lead actors (Renee Zellweger) was from the South. Nicole Kidman (Australian), Donald Sutherland (Nova Scotian), and Jude Law (English) all delivered competent Southern accents.