Music Composers Unite!
There are two ways to approach. First, if you have time and if you have faith in the potential quality of the product (and this is always my preference!) - get involved in the outset. Become not 'the composer' but a TEAM MEMBER whose responsibility lies in being the music department, all aspects. You are the musical expert, you are the person who understands the craft of ways to turn emotion into music, you are the person who hopefully has enough experience in all different kinds of musics and who is free enough to step away from it all and you are the person that will dig DEEP into the film, the characters, the plot, the far seeing implications to society as a whole for this product being created and put out into the world where it changes people. So I LOVE to be able to jump in at the script stage, write themes, motifs, concepts, maybe if the feeling is going to be allowed to build character themes, its a great time to really get inside the characters and their development and try to define them musically. Since its all unattached to the timeline, you can get invaluable feedback from the director on the overall direction of musical concepts long before you ever get to post and begin the scoring process.
Now, actually scoring the dramatic underscore - this should never be done in earnest until you have LOCKED PICTURE, meaning no more edits at all! There can still be post work to be done (color corrections, ADR, foley, etc, etc, but the picture should be locked in terms of number of scenes/frames
Unfortunately unless you are working on a very high profile or high paid gig, it will almost never pay to do the pre-production era work. I dont suggest this as a businessman, even in the slightest, but as an artist and if you personally dont mind and have the time and you are still defining your career, it will pay off tenfold IMO to do the script level writing. James Newton Howard (I think its him at least, or one of the big guys) always does this kind of approach and its something I really love to do