Executive Meddling: For the Walking with Dinosaurs movie - 20th Century Fox decided to add voices to the dinosaurs to help kids get the film, but the decision was made really late in production to the point where there isn't even lip syncing. The majority of viewers disliked the change, as they found the voices annoying and distracting.
Baryonyx and Cetiosaurus were to appear in the main series, but they were replaced with Utahraptor and scrapped entirely, respectively.
The Sea Monsters book describes a number of scenes that didn't make it into the series, such as the female nothosaurs leaving their eggs on the beach at night (there is also a picture of Nigel inspecting those eggs, meaning it was filmed) and Nigel meeting the utterly bizarre aquatic ground slothThalassocnus before going to look for the shark C. megalodon.
Originally Walking With Beasts was going to air first (the idea being that the trilogy would be moving backwards through time), but they only recieved enough money for a show about dinosaurs, and they made Walking With Dinosaurs.