There's one scene on Six Feet Under when Brenda has a friend from Australia over, they take some LSD, and during the scene where they're under the influence, Rachel's Autralian accent is in full force.
I think Kim C. is actually Canadian. I think there are more Canadian actors than Brits and Aussie ones.
Nicolette Sheridan is from England but she moved to the US when she was about 10 so her American accent is probably real. I know a few people who moved to the US when they were 10 or 11 and yet they still speak with a slight accent.
Charlize Theron is from South Africa and supposedly she couldn't speak a word of English when she moved to the states. Don't people living in South Africa also speak English???
I only remember her saying in an interview that since the American accent was 'necessary' for roles, she worked to completely get rid of her natural South African accent, so even in interviews now she sounds American.
Wikipedia says Afrikaans it is her first language, but I somehow doubt she didn't speak any English until so late in life -- Wikipedia also says English is the most commonly spoken language in official and commericial life in SA (just like Singapore!)
If she moved to the US at around age 19 (as Wiki suggests, which would be around 1994/5), and had a speaking role by 1996, somehow I doubt a non-English speaker could be fluent enough and without an accent (or rather, with an American accent), that quickly.
As for Kim Cattrall, Wiki says she was born in England, family emigrated to Canada when she was one, but when her granny was ill, she moved back at age 11, staying till age 16 and attending school before returning to Canada for senior year.
Me? I'm neither Brit/Aussie, nor an actor, so how's that relevant to this thread?
Everyone has "an accent" of sorts, the real question is whether it's different from the one you (or most in your location) have.
I'm a little schizo in my accent. I actually first learned English in England, so my original accent was Brit, public schoolboy if you will. Then I 'returned' to Singapore, where it's been mostly 'lost' in so far as my speaking with any other Singaporean is concerned, but I do speak with less of a "Singaporean" accent than most here. It's still there in the background though, and when speaking with a Brit, or oddly for that matter an American or Aussie, it comes back and I suppose with a bit of work I could pass for native Brit.
I was in Idaho Falls recently, having lunch in a Japanese restaurant, and the sushi chef was half-American half-Japanese, son of the owners. He commented that he found it easier to understand me than half the other guys in his own kitchen...
(As another oddity, because I spent my early years out of Singapore, and my mother isn't Singaporean, I have been told that my Mandarin accent isn't Singaporean either. But I don't know enough about Mandarin accents to say what it does sound like -- some have thought I was Taiwanese, but a native Taiwanese scoffed at the suggestion. And I seriously doubt I sound like a native Chinese either. Perhaps there's an accent for the overseas Chinese diaspora?)
Not celebs, not actors, but I adopted two girls from Russia in 2007. A co-worker who is from Lithuania says that my oldest daughter now speaks Russian with an American accent.
If we are now including actors/actresses in movies then the best performance by a Brit playing an American that I have seen recently is Kelly MacDonald (Carla Jean) in "No Country For Old Men". Her accent was spot on.
And Theron had a fake British accent in Arrested Development.
I've known people who learned English in their high school years, and able to speak it with an American accent. Whether they can switch between their native accent and American at will, that's another story.
I guess some people are just a natural when it comes to languages.
I'm more baffled by how British singers sound American when singing.
Assuming that you're talking about rock/pop singers, the first generation of British rock singers were heavily influenced by American rock 'n' roll -- Bill Haley, Buddy Holly, Little Richard, and of course Elvis. They emulated the sound when they made their own records; I've heard Cliff Richard say that it would've sounded unnatural for him to sing, for example, "Do you want to dahnce?"
I can tell somewhat when a non-American sings but sometimes it is difficult. I have heard that when singing, people use a different part of the brain. This is why Mel Tillis doesn't stutter when he sings.
And why Jim Nabors sounds more like Robert Merrill than Gomer Pyle when he sings.
Melanie Lynskey (Rose on Two and a half Men and Pauline in Peter Jackson's Heavenly Creatures) is from New Zealand.
On the general subject of accents, Isaac Asimov had a funny story he used to tell on himself. He and his wife were watching the great actor Peter Ustinov on a talk show one night, and Ustinov was doing what he often did, telling a long story in dialogue form and playing all the parts (including the women) himself, using different voices. This particular story involved Louis B. Mayer of MGM, and Ustinov, a gifted mimic, did an uncannily accurate imitation of Mayer. Halfway through the story Asimov turned to his wife and asked, "If he can talk regular, why does he bother with the accent?"
Please bear in mind that not everyone in England pronounces it "dahnce". That's (generally) a southern pronunciation. British singing may sound "American" to you because you're expecting it to sound all Cockerney-me-old-china. But the best, or at least the most successful, British music has come from the northern cities (my opinion as a southerner).
Now, where's the thread about all the Irish/Welsh/Scottish actors stealing English actors' jobs with their phony attempts at accents?