The exploits that have occurred in Day-Lewis' eternal quest to remain in character are legendary: he broke his own ribs for My Left Foot, fought random strangers in Rome for Gangs Of New York, and learned to speak Czech for The Unbearable Lightness Of Being, just so he could speak with a Czech accent. Just to reiterate that: The man learned an entire language, just so he could then perform his lines in an entirely different language, but with just a touch of an accent. That's fucking bonkers, no matter how you feel about art and cinema and wacky mustaches.
It's all well and good for Day-Lewis; it's fun to have a really nutty actor out there to win Oscars for us and maybe make some nice shoes while he's at it. But now other actors have picked up that the best way to be truly great is to torture yourself. That's why Jared Leto starved himself to win an Oscar for Dallas Buyers Club, and mailed used condoms to his co-stars to prepare for the role of the Joker. That's why Adrien Brody dumped his girlfriend and ditched his life to be in The Pianist. It's why Jamie Dornan followed a random woman to get in the head of a serial killer for The Fall. If you replaced the second half of most of the sentences in this paragraph with "and then began a police standoff that lasted three days," it would fit just as well.
It's all the same path that made Dustin Hoffman stay up for days (supposedly) to look exhausted in Marathon Man, even though he got thrown shade from Laurence fucking Olivier to JUST ACT. Acting is becoming a war of escalation over what an actor is willing to do in real life, instead of, you know, while acting.