A little bit of meta for your Friday afternoon. I've probably discussed this before - I know I have in comments in other people's journals, but in light of the movie release, I think it bears repeating.
So, in the OotP movie,
right before Sirius dies, he calls Harry "James" - "Nice one, James!" or something to that effect. It bugged me a bit, but I was enjoying the whole Department of Mysteries scene so much, that, to be honest, I didn't really care. But I've seen a number of people mention it in their reviews, not only those saying that it bothered them to varying degrees, but some go so far as to state that "Sirius thinks Harry is James" is cut and dried canon. It's not. Not from my perspective. Not even a little bit.
(The rest of this is strictly book related)
The notion that Sirius can't distinguish between Harry and James has been a huge point of contention since the release of OotP (the book). I've never bought into the idea that Sirius couldn't distinguish between James and Harry. Not at all. My feeling is that he wanted Harry to be more like James, and was disappointed on a few occasions that he wasn't - particularly when Sirius wants to come up to Hogsmeade to visit, and Harry tells him no, afraid that Sirius will get caught. Sirius then says that Harry is less like James than he thought, because the risk would have made it fun for James.
It was Molly who, in her anger, suggests that Sirius was confusing the two, however Molly has previously been shown to have rather poor judgment when she lets emotions get in the way - particularly when it comes to Harry. Prime example is during GoF, when she reads Rita Skeeter's article about how Hermione broke Harry's heart and believes it - sending Hermione a teeny easter egg - yet, at the beginning of GoF, when Rita was writing defaming articles about Arthur, she was very adamant about Rita being a nasty gossip-mongering cow.
In OotP, the scene in the kitchen on Harry's first night at Grimmauld Place, (chapter 5) after dinner when Sirius wants to tell Harry what's been going on and Molly is in uber-overprotective mode to a real fault, she is incredibly spiteful to Sirius:
"He's not a child!" said Sirius impatiently.
"He's not an adult either!" said Mrs. Weasley, the color rising in her cheeks. "He's not James, Sirius!"
"I'm perfectly clear who he is, thanks, Molly," said Sirius coldly.
"I'm not sure you are!" said Mrs. Weasley. "Sometimes, the way you talk about him, it's as though you think you've got your best friend back!"
"What's wrong with that?" said Harry.
"What's wrong, Harry, is that you are not your father, however much you might look like him!" said Mrs. Weasley, her eyes still boring into Sirius. "You are still at school and adults responsible for you should not forget it!"
And then she goes on to lash out at Sirius after being unanimously overruled, even by Arthur, and one of the best lines (if not THE best) Remus has in the whole series ensues:
"Molly, you're not the only person at this table who cares about Harry," said Lupin sharply. "Sirius, sit down."
I used to like Molly - though I was annoyed with her over the easter egg thing in GoF because, um, hypocrite much? - but that scene in the kitchen in particular, where she blames Sirius for not being there for Harry despite the fact that she knows quite well that he was unjustly imprisoned without a trial... I remember reading it the first time and REALLY wanting to smack her.
(I know she means well, but she was vindictive and petulant, and far too overprotective. In the end, it was Harry's lack of knowledge - or limited knowledge - that led to everything spiralling out of control. Not that it was all Molly's fault. Dumbledore played a very large part, as did others. Harry himself, too, in all of his capslock-of-rage stubbornness. Of course, the plot also depended upon Harry's lack of information. :-P)
So, getting back to my original point, I never believed that Sirius confused Harry and James. I think he was disappointed that Harry didn't have more of James' playfulness and the same sense of mischief. Harry breaks rules like his dad did, yeah, but he doesn't do it for the fun, for the thrill - like James and Sirius did. He does it because someone is usually after him, trying to kill him, and he's trying to find out what the hell is going on. Big difference. We never see Harry going for an exploratory late night stroll under his Invisibility Cloak just for the fun of it, and I don't know about the rest of you, but if I'd had an Invisibility Cloak when I was a teenager... *cough*
Also, I think Sirius really sees the impact of the loss of James in Harry's life, moreso than anyone else. They were the best of friends. He can extrapolate what Harry might have been like, what his life would have been like, if James and Lily had lived and he'd grown up with them as a normal child. Harry doesn't have the luxury of just being able to have fun for the sake of fun, and he certainly has no sense of what it means to be carefree. And I think that Sirius not only mourns the loss of James, but also mourns the loss of the child Harry could have been, and perhaps should have been.
Snape, on the other hand, seems to look at Harry and see only James. But that's a whole different ball of wax. :)