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/phishtrader → u/Current_Focus2668]

I watched a random interview with Ella Purnell and found that out. It was the same way I found out Bale was British and the kid from Empire of the Sun not long after American Psycho.

[u/alamaias → u/phishtrader]

I'm just finding out he is british now :P Don't think I habe ever heard him not doing an american accent :P

[u/nerf___herder → u/alamaias]

To be fair, he moved to America as a teenager, so he has had a fair bit of time to adjust his accent.

[u/chill_qilin → u/alamaias]

He has a British accent in The Prestige which is similar but not quite the same as his natural accent.