Thats' the point, they haven't.
But several are known to have been scripted, and in some cases even filmed then cut - for example, scripts for Sunset Boulevard show that it was originally supposed to start in a morgue, but was changed because test audiences giggled. The result of the cut is the iconic 'body in the pool' opening we all know today.
So, um, that sort of thing.
( ,
Thu 17 Jul 2008, 16:33,
archived)
So, um, that sort of thing.