Sherlock knew from the beginning that Moriarty would come after him, and was setting plans in motion to draw him out etc...but I don't think he knew the full extent of what was going to happen. He didn't know he would have to "kill himself" until the moment outside Kitty's flat when he's pacing up and down.

kazza474 wrote:

Sherlock Holmes wrote:

Does Sherlock really look sad when he thinks John can't see him? And if so...why???

No he doesn't look sad. He looks deep in thought.
He's already decided that he will need to fake his death; he is trying to sum up the impact on John, possibly deciding whether to let John in on the plan.
Molly mistakes the looks for sadness. I would say it was more of a case of mixed feelings.
When Molly says she's getting something to eat & does he want anything, I think he was going ask her then about her helping him with the plan but decided to wait to see how things panned out.

Sadness isn't a useful emotion; you cannot change what will be. In its place Sherlock has thoughts of how to 'right wrongs'.

I think he was already planning his fake death, and didn't want John to know about it, let's face it, John's just not that good an actor (remember all that business with telling Sherlock that Irene had 'died'). I also find it funny that John never finds out about this IOU thing, which led me to believe that maybe IOU is something to do with medicine. 

It is exactly as Molly says.
Sherlock's look reminds her of that of her dying father's.
So she knows something is wrong, related to death and she knows Sherlock is upset about John...like her father would have been over his family.