Talking to Access Hollywood, the Tron: Legacy actor applauded Stewart's high level of focus when approaching the project and promised that fans would not be disappointed by the final product.
"Kristen's phenomenal," he said. "She's so focused, especially on this role... she's so amazing and the world is going to get to really see that in this one."
He went on to confess that he feels privileged to have taken part in such a "rare" opportunity to bring the film - based on Jack Kerouac's book of the same name - to life.
"It was wonderful. It was such an incredible rare journey that we all got to take, so I'm very proud of it. My anticipation is just as high as everybody else's about it," he explained.
On The Road also stars Kirsten Dunst, Sam Riley, Amy Adams, Viggo Mortensen and Tom Sturridge. It will hit US cinemas later this year.