Some examples: DiCaprio in "Gangs of New York", Cheadle in "Oceans 11", Norton in "The Illusionist." All these accents were so ridiculously bad they take away from good films.
Surely the directors can hear how badly they sound on set?
I guess they're shitty directors. Its certainly something that can/could be picked up in casting, or worked on in pre-production & rehearsing. Most actors these days have dialect coaches. Some times, the draw-power of the actor is such that they put up with sub-par capability in that respect.
"Just let him be British. Him being an American isn't really important to his character."
I agree, especially in today's world. People are on the move all the time now, so it's natural to hear different accents.
Back when Mary Poppins was made I suspect few people were aware of what an actual Cockney accent sounds like. Even today, some people don't seem to realize how many British accents there are.
Perhaps they felt the performances had qualities that compensated for the bad accents. In many cases, bad accents don't matter to most people -- only when you notice them, which is usually when it's one you're more personally familiar with.
For example, I've heard so many American actors doing bloody awful British and Irish accents (not just Mr van Dyke) but -- of course -- it's only the British section of the international audience that is immediately focused on that and taken out of the performance a little by it. Presumably, it's the same with other accents.
In the case of someone like DiCaprio in Gangs of New York, there's an element of star power involved. Gangs of New York probably wouldn't have been financed to the same degree without DiCaprio on board (or someone of similar box office pull at that time). So you'd probably rather have DiCaprio with his dodgy accent than someone who could do a better one but wouldn't get bums on seats. See also Kidman and Cruise in the borderline-insulting Far & Away.
All that said, I think these days Hollywood is more sensitive towards these matters than it has been historically. Because, I think, audiences are more tuned in to these things... And overall accents have improved.
Oh yeah. I'd forgotten Diaz's. You're right. They're both pretty bad. Now you mention it, I think hers is worse than his.
I do think Hollywood has changed recently in this regard. I do think they try harder -- through dialect coaches and whatnot -- to get accents right. But some actors have an aptitude for it and some don't, so you still get some absolute stinkers... sometimes in otherwise good performances though.
He was on set of a movie he was doing with Brad Pitt and after they met, Pitt asked him, "How do you like my Irish accent?" in his attempt at the accent. Ford responded with, "When are you going to show me it?"
I guess things were tense between them the rest of the way. LOL. I don't know how true the story is, but that sounds like Ford.
"Surely the directors can hear how badly they sound on set?"
If they're not native to the home of the accent that's being faked, then probably not.
For the most part the only bad accents I ever notice are bad Maine accents, because that's where I'm from. Bad Maine accents are annoying, but fortunately, Maine isn't a common setting for movies and TV shows. For the record, I've never heard anyone convincingly fake a Maine accent, neither in movies / TV shows nor in real life. And native Mainers who don't naturally have a Maine accent can't fake one any better than e.g., Tom Bosley from Chicago did on Murder, She Wrote.
If I were directing a movie set in Maine, and actors with real Maine accents weren't an option, I'd want actors with a neutral American accent, which is perfectly valid for Maine. Accents here vary from prominent Maine accent to no Maine accent at all, even within a single family/household. In my case, my mother, late father, and older sister all have/had a prominent Maine accent, while my older brother, younger sister, and I have no Maine accent.
There aren't many stars from Maine, but as an example, listen to Anna Kendrick talk. She's from Portland and doesn't have a Maine accent. The same goes for Rachel Nichols who's from Augusta (she starred in P2 [2007]).
Terrible, but I still liked him in that movie. I liked him even more in My Cousin Vinny (1992). I don't know if he was trying to do an Alabama accent in that movie or not. If he was, I'm in no position to judge it.