Groot is such a weird character and one of the hardest parts about bringing him alive is the fact that he only says "I am groot." With that one line you need to say a million things, to bring what is actually a very dynamic character to life. Here you have an extremely intelligent, very long lived, deeply injured yet very heroic creature and to express his thoughts and emotions you are limited to 3 words. This is the task.
James Gunn was interviewed on the Pointless Podcast and revealed a little more about Vin's part in the movie.
All of the I am Groots that were earlier voices didn't sound very good at all, they sounded like s#$#!. It just didn't work. Then Vin Diesel came in and, in one day, laid down all these 'I am Groot' tracks, and he's a perfectionist. He made me explain to him exactly what (Groot) was saying (before each delivery).
|Vin laying down the "I am Groots"|
Vin Diesel could have just added his name to the movie shown up and said I am Groot a few times. It sounds though like he came in and brought his A game. I have noticed that Vin Diesel going all the way back to the first Pitch Black has had a unique voice that can at times feel somewhat otherworldly and haunting. I am really looking forward to hearing it.
Vin Diesel and James Gunn also apparently have created an I am Groot language where Groot's meaning is different by his inflections in his voice. This being similar to what Maximus the Mad says about how Groot speaks in the comics. The fact that this is even being attempted is amazing to me. James Gunn has also revealed that only Vin Diesel and himself have access to the special Groot script that has the translations.
Only Vin Diesel and I have the special Groot script with translations of what he’s saying. https://t.co/W9loN1IcJF— James Gunn (@JamesGunn) November 12, 2015
More Groot and Marvel News:
Who is Groot?
Guardians of the Galaxy character HUB
Guardians of the Galaxy Easter Eggs
Vin Diesel kind of fell into the roll of Groot. The story of is outlined already:
How Vin Diesel became Groot