Such a brilliant screenplay. I'll leave it to the serious historians as to who contributed what, except to say I feel the original play provided more than has been acknowledged many times, and it all comes together brilliantly.
The dialogue jumps off the page, the characters are so beautifully sketched: not just nihilistic Rick and joyfully corrupt Renault, and brave (yet silly) Laszlo and wet Ilsa (let's be honest, she can't even think for herself)... but the smaller ones, like drunk and trashy Yvonne, poor Annina, joyous Sascha, the pickpocket. Buffs never give that much love to Carl, Sascha and Sam but they provide a lot of warmth to the movie - their concern for each other and Rick, etc enables Rick to be hard boiled and tough but still likeable because he looks out for them.
The letters of transit is a brilliant Macguffin (in the draft I read signed by Weygand, not de Gaulle... Weygand makes more sense), the action is logically followed through: death of the couriers, Strasser arriving, Ugarte, Rick having the letters, Laszlo arriving with Ilsa, setting up Laszlo wanting the letters and getting involved in local politics, the clash between Renault and Strasser.
I love it how clever Renault is - taking bribes, enjoying hanging out with Rick, knowing Rick's got the letters, knowing it was Ugarte responsible... even at the end Renault almost outsmarts Rick by calling Major Strasser, then at the end doing the final wonderful double cross of the Axis. His love story with Rick is a lot more effective than the one with Rick and Ilsa because Ilsa really is a bit of a ninny, just going with the flow. She can't even shoot Rick.
It's very adult but the writers get around the censorship superbly. Look at the scene where Annina checks with Rick whether Renault will keep his word about getting a visa - it's all implied (she'll have to sleep with him to get it, she doesn't want to tell her husband), and so beautifully done. There's also the "Le Marseilles" scene - which actually doesn't read that well but of course played superbly.
It's just so, so good.