I'm always suspicious when someone makes a definite statement about accents. They simply aren't that clear cut, so to say. And an individual can and will vary his accent according to circumstances, assuming they're brighter than a 40-watt light bulb. One neither wants to sound like one is uneducated or putting on airs either. But some things are hard to cover up and other things are difficult to imitate. The thing is, an accent is a complicated thing. It includes not only the way various words are spoken but the tempo of speaking and the vocabulary itself. It generally won't come through in written communication, mainly because most people don't write the way they speak. Accents vary not only by locality but by class and education and I suspect by occupation to some extent, at least with vocabulary. I also believe that sometimes people will intensify their accent (accent their accent) for the humorous effect. And obviously, people will pick up an accent a little, which might be more accurately described as losing an accent. It's something that happens when you move somewhere else.
Then there's the accents of non-English speakers speaking English. It's something you hear in the movies but it isn't always accurate but of course, they won't all have equal command of English, either. We have a relative who has a most interesting Hollywood Russian accent, only she isn't Russian and doesn't even speak it. She's Serbian.
One curious American accent is what might be called Southern coastal, in the Chesepeake region and further South. Some say it sounds Elizabethan, though I have no idea how anyone would know. But some words are pronounced in an odd way, like "dog," pronouced with a long "o." Little things like that are hard to catch unless you're listening for them. Once, in college, 45 years ago, an Englishman going to the same school asked me how I liked my 'state car." We had a little session of twenty questions before I realized how he liked my "estate car" and was referring to my Land-Rover station wagon. I liked it very much, thank you!