In fact the entire opening scene(s) are done better in the movie than in the book.
In the book John Carter had, at first, only one cause - the cause of Dejah Thoris. He tells her that he loves her, but wants nothing from her so long as he is her only source of possible rescue. That is quite heroic enough in my mind - rather than the invented cause that the Helimites were saving the planet while the Zodongans were destroying it.
John Carter was never the reluctant warrior. He would often throw himself into combat for no other reason than that he saw one skilled (and presumably heroic) opponent about to be overcome by several (who were cowardly teaming up to bring down the better man).
let’s start with .. . BEANS.