Casablanca is the most famous example. The script was being written literally while filming was well under way, one scene at a time. No one knew how the story was going to end until the last page was shot. It's amazing how well the story holds together, as there was virtually no "story" at all to begin with.
I'm sorry, but that's not at all true. Casablanca was based on the play 'Everybody Comes to Rick's' by Murrary Bennett and Joan Alison, which was a rather somber political piece. It was being furiously adapted on set to turn it into a Hollywood romance/ drama, and to punch up the dialog. While it's true no one knew if Ilsa would end up with Rick or Lazlo until they shot that scene, that's because no one was sure if they'd go with the play's ending (which they did - and which they were afraid would be a downer), or if they'd create a "happy ending" for the star-crossed couple. I've been asking around - so far the only movie anyone's come up with that began with just a star and a concept is Top Gun. So it happens, but I think it's relatively rare.
I love Rachel as an actress, but upon thinking it over I don't know if she would be the right pick for Catwoman. She looks a lot like Kaite and Maggie. Her as Selina Kyle would no doubt remind Bruce Wayne of Rachel. Then again, that could be the idea.
I'd pay to watch Rachel Weisz recite words from a dictionary. She's absolutely beautiful. This rumour is doubtful though, probably just hype-building or her agents at work.
I have to say that I think they should go with Ashley Scott for the role. She played the daughter of Batman and Catwoman on Birds of Prey. I think she could be a good choice.
Selina needs to be more than just a pretty face, she needs to be dangerous and playful and I don't think Weisz has that in her, at least not in any of the roles I've seen her in. Personally, my first choices would have been either Salma Hayek or Rosario Dawson. As for Halley Berry; although I never bothered watching that Catwoman film (trailer made it look crap) I think given the right film she could do the character justice. I'm just waiting for an X-Men Origins: Storm movie to be announced, anyone else notice here character in the Wolverine trailer? Regardless, anything you hear about anything Batman related in the next six months or so is most likely going to be either completely fabricated or pie in the sky. I'm sure that as soon as DK started breaking records, hundreds of script writers began tapping out screenplays and treatments by the ton for any property even remotely associated with the Batman franchise.
If the character is successful in the next movie, they're going to give her a spinoff: Batman Begins Origins: The First Catwoman
Dawson's the first name I've seen mentioned that I would get excited about. Most guys are just naming whoever they'd like to see in the catsuit. But Rosario Dawson would give the character a turn worth seeing. Though I'm fairly against the idea of Catwoman in the next film at all.