Shamus is shown to be kind towards the Banks family, Mary Poppins and Jack. He even praised the group for helping fix the wheel on his carriage. Just like Clyde, he is able to talk. He speaks with an Irish accent.
Shamus is described as a dog who is part poodle when he introduces himself and like all the animals in the animated sequence, he is anthropomorphic.
When a fight over their late mother's Royal Doulton Bowl accidentally leaves a crack in the family heirloom with Shamus and Clyde telling that the Banks children had broken it, causing their carriage wheel to break, Mary Poppins takes the Banks children and Jack inside the bowl to help fix the carriage wheel. When the children asked for a ride because they felt the trip was too short, Shamus and Clyde give the group a trip through the bowl to the music hall.
Later, when Georgie gets kidnapped by the Wolf and his accomplices, both Shamus and Clyde help drive them get on to the wagon carrying their brother. Fortunately, the children manage to do so.
- O'Dowd also portrayed Richard in the Marvel Studios film, Thor: The Dark World.
- O'Dowd was originally going to voice Tigger in the Disney live-action film, Christopher Robin as this would have been his first Disney role in 2018 while Shamus would be his second role in the aforementioned year. However, O'Dowd's test voice for Tigger was negatively received by fans for his participation as Tigger; as a result, Jim Cummings replaces O'Dowd in the role of Tigger in the final version of the film, who also voiced Pooh in the aforementioned film.