I've been here at Pixar a few years and I've seen how movies are made here. And it's not a secret. In fact it's in all the Pixar "making of" books. The director has this story that he/she wants to make and it all goes from there. Other studios have their own tried and true ways of making movies.
Though exact details of the script were being kept under close wraps, Stern's new take is expected to advance the "Toy Story" franchise by taking the characters on the road and out of Andy's room. Stern is also understood to have invented a couple of new characters for the next installment.
The studio is expected to hire another* screenwriter to write a polish on the script.
(* my bold. I does what I likes)