According to i09, Vin Diesel has released behind-the-scenes Guardians of the Galaxy footage of him learning to walk with the stilts. Also the video tells us that Diesel will only have one line in the movie.
Excerpt From Video:
My Father taught me there are no small roles, only small actors. I have one line in the movie, but that doesn’t mean anything. I’m still going to make it the best thing I possibly can. Even if it means taking Andy Serkis to a whole other level. Which is what we wanna do with our motion capture.