You can have a great script, or a great director and a bad script, and get a great movie. Nothing really guarantees anything.
Sometimes things are nothing on paper, but a genius director turns it into something amazing.
More often than not, if you've got a good director and a good script you can't really go wrong.
When I do go to L.A., it is usually for a reason - to meet with a director or something - but I'm always so happy to go back to London.