- First-time viewers might not catch this, but remember the part where Sophie travels back in time and she yells at Howl to "find her in the future?" Remember what Howl's first line to her is?Howl: I've been looking everywhere for you.
- In the book, the antagonist Miss Angorian claims to be engaged to Wizard Suliman. Similarly, the movie's antagonist is named Madame Suliman. So really, Miyazaki's choice to give that character that particular name was pretty clever.
- In the book, Sophie has two younger sisters, Lettie and Martha. In the movie there's only Lettie, but a girl named Martha is mentioned at the beginning of the movie. While Sophie is watching Howl's castle from her window, the shop girls can be heard gossiping about a girl named Martha who had her heart "torn out" by Howl. Later, after Sophie becomes cursed and goes out into the Wastes, the man warning her of the danger says to his wife that she "says she's looking for her youngest sister".
- Why does Calcifer like to eat everything? Because he's fire! And fire consumes everything!
- In the book, when a duelist comes to ask for something to give him an equal chance in a duel, Sophie tells the powder she gives him that it will "make it a fair fight". Since she can talk life into things, it probably really did make it a fair fight!
- Why is the house of a wizard as vain as Howl so messy and dirty? Well, for one, he is away more often than not, and he gets back dead tired most of the time so obviously he wouldn't be bothered even if it hurts his vanity. And on the first morning, Markl/Michael tells Sophie the fire is off limits when Howl is away since Calcifer only obeys Howl. Even a task as simple as washing the dishes or washing up needs hot water and then some.
- When the king walks in on Suliman and Howl, the latter being in disguise as the king, he's unfazed, then impressed that this was "the best double she's made so far!" As he seems to be deeply involved in the war efforts, it would make sense that Suliman would be responsible for making endless body doubles for the king, whose lives would be at risk of assasination. Impressive, then that Howl apparently made the best double yet! It helps that he was her best student.
- In the book, Sophie makes very much of the fact that she is the eldest, and though Martha is only her half-sister (Fanny being mother to Martha and stepmother to Sophie and Lettie), there are three children in the family- therefore, according to the rule about youngest children, Sophie is doomed to spectacular failures and Martha is fated to succeed and be powerful. It turns out that she is Wrong Genre Savvy, but the hints about that start practically at page one: According to these rules, you would expect Fanny to be terrible to Lettie and Sophie and to dote on Martha, but she does her best to do right by them, even though she never considers what any of the girls (Martha included) really wants. You would also expect the sistery to bully each other but they don't do that either. Clearly if these "basic" rules don't hold, then any child can find their fortune, regardless of birth order- and indeed at the end they do.
Fridge / Howl's Moving Castle