I love the whole little Walter incident, but almost as much as the actual saving of poor Anne's back, I love Frederick's reaction to finding himself alone with Anne for the first time:

"The surprise of finding himself almost alone with Anne Elliot, deprived his manners of their usual composure: he started ... he walked to the window to recollect himself, and feel how he ought to behave. " (ch 9)

He has been in her company many times now and has his routine on how to deal with her down - mostly by ignoring her. But now he's alone with her. He must say or do something, but he doesn't know what. He is surprised and he has to think about it. It doesn't take this much effort and thought to speak the ordinary pleasantries to someone you doesn't care about; Someone who has no place in your heart. It's a little moment, but very telling, I think.