What are the pros and cons of writing a book and screenplay at the same time for the same story?

You should write your book first and then make it into a script. If you can, try to get a copy of an old script. I am lucky enough to have a friend in the movie business and got an old script from him. It's not as easy as you think to do a script as you have to add in (actions of the person at the time of their lines in the movie.) Example: A woman is at a bar having a drink with a man and her line is "I'd like to get to know you better. How about coming back to my room?" In brackets you have to put something like, "Amy crosses her long, shapely legs and leans so close to Rick that he can feel her hot breath on his face. Rick reacts by putting his hand on her leg and looking her straight in the eye and telling her it will cost her!" Many good books are made into scripts and seldom scripts are made into books. I love to write too and want to do a script, but I'm tackling the book first and know that if my book is picked up by a publisher I want to have the contract read, "Should any movie options come forth from this book, then the author shall receive a percentage of the script deal, plus a percentage of the total take on the movie once released and also any advertising products that may follow." Harry Potter is a prime example.