Joyce has a keen eye for the public and excels as she creates a cast of characters who are all too realistic next to Harold's quasi-spiritual pilgrimage. While everyone else wants to sanctify him we begin to see him in a new light—a light which shows his human failings while only making him more endearing to the reader.
In other words, Joyce has created a work in the epic tradition of the quest. From the outset of literature no theme has been more prevalent. From the Odyssey to Huckleberry Finn to Marilynn Robinson's Gilead, the quest for something more is a reflection of a human longing. Joyce has created a character we admire because in part we want to be him. We want to seek out on a new journey with only our wits to save us (gasp--he does not even have his cell phone!). We want to meet physical, emotional, and spiritual dilemmas and push through them. We want to be challenged and then succeed beyond our own expectations. Like any good quest, the Harold Fry we meet at the outset of the novel is not the Harold Fry we say goodbye to at the end.
Much of what happens or is revealed toward the end of The Unlikely Pilgrimage of Harold Fry alters your perception of what has occurred, so I'll resist any temptation to spoil the plot for you. If the book has any success you can count on a movie. Joyce is a playwright for the BBC and much of the book could be dropped on screen as is. This is not a critique of it since as a novel--it still works.