Why is Vin Diesel looking down? Is he worried he’s gonna forget his line?

Vin Diesel asked the director his inspiration for every line he did and did multiple takes until he was satisfied. He also recorded the line over 1,000 times and also recorded his lines in Mandarin, Portuguese, French, and Spanish so they could use his real voice in those versions. He’s looking at his lines because Vin Diesel is a  dedicated  motherfucking professional. 

Vin Diesel made you fall in love with a character who said 4 words. The CGI brought his words to visual life, but they’d be meaningless without his amazing command of voice. 

Basically vin Diesel had the challenge of:
"OK, this is the message you’re trying to convey"
"Alright"
"But you can only ONLY say I am Groot”

Because every time Groot says “I am Groot” he means something and vin Diesel had to convey that message as best he could with only those 3 words through inflection, emphasis, and emotion.

And this was his first job after suddenly losing one of his closest friends.