You're right, Therese. The best casting director in the world can never match everyone's imaginations. But my problems with PoA were with the script. It's the turning point in the series and Steve Kloves took the book, ran it through a shredder and stuck it back together, adding more pointless business for Hermione while leaving Ron as a dundering fool. When they gave Hermione Ron's best line ("If you're going to kill Harry, you'll have to kill us too!") I actually swore out loud in the theater.
And just for the record, my Lupin was Colin Firth.
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