I have two different experiences in this regard.
Experience 1 : QF 81 SYD-SIN seat 24K bulkhead aisle, shadow on window. After take off, a lady sitting in 24G taps me on my shoulder and asks if anyone is sitting in the window seat. I hesitated and said no (I mean, I can't say, the flight has already took off). She then asked if she can sit there because she'd like to see the view outside the window. She told me that she was leaving AU for good, going back home to Italy. I couldn't say no ... so she sat in the window and immediately dropped down the shades .. I was thinking "you wanted the window BECAUSE you said that you'd like to see outside the window" !!! She kept the shades down all the way until SIN, when the crew asked the shades to be open for landing.
Experience 2 : CBR-SYD. I think I was in the bulkhead once again, the window was empty. After take off a young girl asked if she can sit in the window. I couldn't say no. So she settles herself in, the FA promptly walks over and asks her to move to her original seat as on the BP. No explanation provided.
The commonality here, in addition to people trying to move around the cabin after take off, is that I'm not able to say no ... surely something for me to work on ...
So, dear AFF'ers, how do I say no when someone asks I they can sit in my "shadow" ? Any suggestions pls?