So.much.word. I just saw TDK last night and I've tried to explain to people what watching Heath as The Joker was like and you nailed it. Absolute perfection.
Best Casting:
Joaquin and Reese in WTL and Ellen Page as Juno (all three have been mentioned but they're big ones for me). Also, Reese as Elle Woods in Legally Blonde.
Julia Roberts in Pretty Woman. I haven't loved everything she's ever done but she was so charming and... warm in that role.
Jake Gyllenhaal in Donnie Darko. Jake has those big, haunting eyes and he just owned that character and film. It's bizarre, of course, but he really pulled me in and made me care.
Anna Chlumsky as Vada in My Girl. I watched this tonight for the first time in years and she absolutely kills me. Aside from being one of the most adorable little girls on the planet in that movie, she delivers an amazing performance. There was a depth to that character, even at such a young age.
Worst Casting:
Rachel Bilson in The Last Kiss. Horrible. I think it was mostly just a poorly written character but a stronger actress would've been able to give the character some other level of emotion or humanity. It came across so, so bad.
Kristen Bell in Forgetting Sarah Marshall. I wasn't at all impressed and I feel like she was the weakest part of the film. Again, I'm not sure who I picture instead of her but... her comedic style, so to speak, didn't mesh with the others. A lot, if not all, of her stuff fell really flat for me.
Kevin Costner in The Bodyguard. He looked stiff, uncomfortable... bored? I don't even know but I watched this on TV at like 2 AM the other night and it popped into my head instantly when I thought of bad casting. I'm not sure who would've pulled it off but he wasn't it. Bad, bad, bad.
Brad Pitt in The Mexican. It's a pretty awful movie (though I did adore Julia and James Gandolfini, hee!) but Julia's character was at least believable (IMO). Brad's performance felt phoned in, he couldn't pull off that character, and it was just a whole lot of bad.