Twenty-two different dogs played Marley.
Owen Wilson's real-life parents played his parents in the movie. His mother's biggest problem was remembering not to call him Owen.
As a puppy, Marley runs into the screen door, then breaks through it. For the initial run-in with the door, trainers released the puppy a few inches from the door so that it would merely bump into it without hurting itself, and then called it to the other side. The puppy had been trained to run through a special "breakaway" screen door, which was built to give way with the slightest pressure.
For scenes in which Marley chews on items, such as the kitchen floor, couch cushions or car seats, trainers cued the dog to "get" the objects. One particular puppy and one particular adult dog were specially trained to chew on things and spit them out without swallowing anything.
Whenever either the puppy or the dog is seen performing such mild action as sitting or lying down, walking (on- or off-leash), playing tug of war, or tipping over objects or biting at them, trainers placed the dog on its mark and stood off-camera, using hand signals and verbal commands to cue the action, which the trained canines were accustomed to. Any object Marley is seen to knock over was a lightweight prop that trainers deemed safe. For shots in which the dog pulls on its leash, off-camera trainers excitedly called the dog to come, causing it to go to the end of its leash and pull.
John Grogan: The book's author has a cameo appearance as the Cocker Spaniel owner in the dog training class.
Dave Barry: During the surprise birthday party scene you can see the former Miami Herald columnist standing on the right of the shot smiling.