He often stays in character throughout shooting -- to the extreme -- so if his character is an arsehole, he probably will be too. I don't think he really plays himself, in fact I think his role as Daniel Plainview was one of the biggest transformations I've seen on screen in terms of stepping inside and acting out another character. To say "He is an angry actor, and the character is angry so he is just playing himself" simplifies it way too much, because Daniel Plainview was not Daniel Day Lewis at all. When I showed my friends some clips of him after the movie they were shocked at how different he is (not only in appearance).
He certainly doesn't play it safe with his role choices, staying in a severely disabled position for weeks on end (breaking ribs in the process) for My Left Foot, and living off the land camping, hunting and skinning animals for Last of the Mohicans. Whether he is an arsehole in real life or not, I have huge respect for him as an actor. But I think he appears pretty humble and very intelligent in the very few interviews with him I've seen. I imagine he is just hell to work with on set, staying in (often ugly) characters. At one point he asked the crew to verbally abuse him and throw cold water over him throughout shooting.
I haven't seen Nine yet (I was initially looking forward to it), but I imagine the movie didn't click because it unfortunately doesn't look to be very good. But I won't pre-judge it too much yet.
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