[jkaunisv1]

I usually get the impression people aren't really trying to match an accent. They're just trying to pronounce the letters and syllables as they would in their native language. Hence the stereotypical cringe-inducing "bonn-joor!" or "watashee wah soo-miss-oo dess-oo". Maybe it's because they haven't heard enough native pronunciation of the words (just like I mispronounce words I've only ever read in a book).

When I speak foreign languages I sort of subconsciously imitate characters I've heard speak in movies or shows. I'm always kind of worried my Japanese will sound like a mix of Naruto and Miyazaki characters.

As for regional accents, I find that's more a matter of knowing the little differences in certain words, like extending or shortening syllables, or rather strongly altering the key vowel (eg. Québecois 'nawn' vs. Parisian 'nohn' for 'non'). As you said, has a lot to do with mouth shapes.

For a lot of regional accents, I find you can fake things by transposing certain vowels. eg. for Australian transpose all the eh's (eg. get, head) into i's (eg. git, hid) and using soft r's (git me a beah mate). Whereas in my limited experience New Zealanders have a sharper change from e's to i's - it's almost "geet me a beah". And of course knowing the local slang - adding 'mate' to the end of your sentences just sounds more Australian.