"There was nine months of treatment writing before I even started on the draft. That’s the hardest work in many ways."
Which means: "MOST of my writing is done while writing a scriptment – which, in this case, took months and months. It’s seventy pages long – the majority of a script, with every beat and even a lot of dialogue. I then turn that into a script, which is written in screenplay format, with scenes, etc. What I’ve been doing the last 48 days is turning the scriptment into a screenplay."
And it has to be good. How do we know? Chris Pratt himself confessed the script moved him to tears. And Gunn corroborates: "He cried when I told him the story I was about to write".
GotG2 will see the return of the main cast

Indeed, you can take that off your chest: GotG2 will see the main cast returning. So Chris Pratt will reprise Peter Quill, Zoe Saldana will come back as Gamora, Dave Bautista will deliver Drax again, and Vin Diesel and Bradley Cooper will do voiceover for Groot and Rocket respectively.
In James Gunn's words "Yes, the main cast is all coming back, along with a few others from the first film". One of the "few others" also include Sean Gunn, who will portray Kraglin for the 2nd time.
