Ingrid Bergman plays Ilsa as being in love with both Rick and Victor not because she's a great actress (which she was), but because she honestly didn't know which man Ilsa ended up with. The dramatic twist ending ("Round up the usual suspects") was the most expedient way to end the film.
This was the studio system era where a studio would knock out 52 movies a year with whatever talent they had on hand. The cast was whoever Warner Brothers had available, and they had lots of different ideas of who should play whom; we just got lucky. They used a staff composer and got Max Steiner (Gone with the Wind) and the signature "As Time Goes By" only stayed in the movie because there wasn't time to write a new piece of music -- 52 movies a year means you keep on schedule.
It turns out that Casablanca is one of the greatest films of all time not because someone worked hard to create a masterpiece, but because of random numbers and volume. It's like the idea that an infinite number of monkeys on keyboards with infinite amount of time will eventually randomly type Shakespeare's Hamlet. Enough random banging at the studio and you get Casablanca.
And here we are with the Internet and everyone posting their random thoughts, pictures, and films. There's some amazing stuff online and more amazing stuff coming down the pipe if only because the sheer volume of creativity that's being captured and distributed.
Sure, 99.999999999% of it is useless crap, but I'll argue that in the next few years we're going to see some work of art come out of nowhere, and the only reason we'll get to see it is because we live in an age where there's so much content that quantum physics comes into play for the next great masterpiece.