Hard to see Nicholson in this and in "Five Easy Pieces" as the same person
I mean they just don't look the same, by a lot. Some people age and, except for a few wrinkles. look the way they did when young, and some change so much they're almost unrecognizable. Al Pacino is another in the latter category.
Anyway, as many know, this is a remake of a Chinese film called Infernal Affairs, some would say a ripoff of it. IMO, the Chinese film is better, especially in its lack of wall-to-wall music, a Scorsese trademark.