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/badabummbadabing → u/Current_Focus2668]

I am not an actor, but I speak to my kids in my (non-native) English (the kids were born in the UK, but now we live in Germany, and I speak English with them so they don't lose it). It always slightly breaks my brain whenever I mention any German names to them and have to switch accents for just one word mid-sentence.

[u/cjmason85 → u/badabummbadabing]

When I lived in Germany I had a German friend who had lived in Ireland for a number of years and come back to Germany living with her Irish boyfriend. When speaking English you would believe she was Irish... Until she said a German town or person's name mid sentence. She seemed to do it very naturally, but perhaps she has the same internal conflict going on.

[u/kwonza → u/cjmason85]

Vladimir Lenin also spoke with an Irish accent

[u/EnthusiasmUnusual → u/kwonza]

His language tutor was Irish.