by Kathryn A
After they left New Earth, everything was fine -- wasn't it?
Spoilers: all of New Who season 2
When a thought whispers in your mind, you assume that it is your own, especially when it is not so very different from the way you think, from the way you used to think. When water washes away mud, it carries the mud with it.
The Doctor had been affected too, but he threw it off more quickly. Cassandra fit less well in the crevices of his mind; because he was male, because he was alien, because he was old. Most importantly, he knew how to deal with the echoes of other minds.
Rose could not distinguish the shadow from the self.
If someone had pointed out Rose's odd behaviour, she would have laughed. She wasn't anything like Cassandra, obsessed with her appearance (but being obsessed about the Doctor was perfectly normal). She didn't think of people as objects (but winding up Queen Victoria, what a lark!). She didn't think she was above the common herd (but she was so special, because the Doctor had picked her). She didn't think the universe revolved around her (but how could Mickey leave? How could her father disown her? How could the Doctor make her stay behind?)
Alas, saddest of all, nobody noticed.
I wrote this because I loathed the giggly way the Doctor and Rose behaved in "Tooth and Claw" and was disgusted at the way Rose's character degenerated this season; and it suddenly occurred to me that perhaps there was an in-universe explanation...