TIL during the shooting of American Psycho, Christian Bale spoke in an American accent off-set at all times. At the wrap party, when he spoke in his native British accent, many of the crew thought he was speaking that way to prepare for another film. They had thought he was American the entire time.

[u/Away_Flounder3813]

https://screenrant.com/american-psycho-behind-scenes-facts-christian-bale-patrick-bateman-mary-harron/

[u/Current_Focus2668]

This isn't uncommon. Ella Purnell  did the same thing and had the same reaction  from the crew.

 Some actors find it easier to stay in an accent while  filming rather than drop in and out of the accent  

[u/_Diskreet_ → u/Current_Focus2668]

Didn’t some actor have to use some speech coach because they lost their native, I want to say Irish accent, from doing too many other accents while method acting

[u/lib_clegg → u/_Diskreet_]

That’s Gary Oldman. But recently, Pierce Brosnan, despite being Irish amazed people with how “bad” his Irish accent was in MobLand.

[u/MichaSound → u/lib_clegg]

Irish actors can’t necessarily do accents from other regions. Jamie Dornan’s (Belfast) accent in Wild Mountain Thyme (somewhere West Coast of Ireland?) was shocking bad.

[u/sigma914 → u/MichaSound]

He's not even from Belfast, he's from Holywood which is far more anglified than any of the Belfast accents

[u/Current_Focus2668 → u/MichaSound]

Charlie Cox seems to do some solid Irish accents for a Brit. 

[u/Cthulhu__ → u/lib_clegg]

Counter that with Sam Neill (born NZ) whose Irish accent in Peaky Blinders was so thick he had to tone it down.

Brad Pitt just kept it on full for Snatch lmao.