The actor stars alongside the 27-year-old beauty in new movie 'Gulliver's Travels' as regal love interest Horatio and found being believable as a princess' boyfriend much more difficult than getting in shape for the role.
He said: "Here I needed to be fit. They got me a trainer, which is very different to what I am used to.
"I have hand-to-hand combat, and then I also have to be someone Emily Blunt could be attracted to. In fact, that's been the toughest thing of all."
Jason also admits he feels grateful to Jack Black, after the actor-and-rocker fought to get him cast in the movie.
He explained: "I love Jack. He gave me my first writing job when I was 20 years old on a pilot called 'Black Market Music' that Seth Rogen and I were writing.
"Jack fought really hard for me on this movie and I owe him a lot."