He is always in great shape and I noticed he never took off his shirt.
It looked pretty real to me.
I read where he'd gained weight specifically for the role..
I watched this solely for Mortensen, I just wanted to see Aragorn driving a black pianist around.
No. He gained weight.
He gained the weight specifically for the role. He later stated that he had to take it off which was nowhere near as fun as gaining it.
I remember at least one scene in which he was lounging in a hotel room in a sleeveless t-shirt where it was evident the pudginess was the real thing.