In the film adaptation, he was voiced by an uncredited Ron Perlman, who also played Dieter Reinhardt in Blade II, Slade in Teen Titans, Bular in Trollhunters, Xibalba in The Book of Life, the High Mage in the 2003 Teenage Mutant Ninja Turtles, the Lich in Adventure Time, Kago in Tarzan II, Warhok in Kim Possible, the Stabbington Brothers in Tangled, Fire Lord Sozin in Avatar: The Last Airbender, Drekk in Men In Black: The Series, Clayface in Batman: The Animated Series, Rumor in The Batman, and Mr. Grasping in An American Tail: The Treasure of Manhattan Island.￼
From the beginning, he was probably just an average Goblin that, like other members of his species, joined the Ogre out of fear. Wormrat probably assisted in destroying the Spiderwick Mansion with the rest of Mulgarath's Goblin Army in the search of the Arthur Spiderwick's Field Guide. However, he wasn't spotted by the Grace children when the giant Ogre and his servants gathered up the weapons the Dwarves had hidden for them in the Old Quarry. When Hogsqueal, Thimbletack, Byron and the three siblings went to rescue both the Field Guide and the childrens' mother, who had been kidnapped by Mulgarath. Hogsqueal, pretending to have captured Jared and Mallory, walked into the Junkyard palace, creating a distraction while Simon rode on Byron the griffin. Wormrat and his Goblins met them, with him ordering the children tied to a broken car while he reported to his master. However, the children escaped, fought off his Goblins, and made it into Mulgarath's palace. It's unknown what happened to Wormrat after Mulgarath's death, but he probably fled with the rest of the army.
During the film, Wormrat (also known as Red Cap) is portrayed as Mulgarath's second in command. He is first shown in the Goblin Camp after his Goblins captured Simon. Since they were supposed to capture Jared, Mulgarath angerly berates Red Cap, ordering him to find both Jared and the Field Guide, and then kill the humans. He and a small band of Goblins then chase Jared and Simon through the woods and to the SpiderWick mansion (with one of the Goblins managing to bite Simon in the calf). Red Cap engages in a brief duel with Mallory, with her managing to cut him across the left eye before Jared pulls her into the circle. Red Cap and the rest of the Goblin Army then surround the house, with him vowing to kill them once they leave the circle.
|232px|Red Cap and his goblins before the stove explodes.]] The Goblins then follow Jared and Mallory as they try to reach Lucinda and Red Cap succeeds in getting a few pages from the Field Guide. Unfortunately for him, most of the pages prove to be worthless, much to the anger of Mulgarath. However, before the Ogre can kill Red Cap, he discovers one of the pages reveals the charm for the protective circle, which allows them to break the circle. The Goblin Army swarms the house, however the humans are armed with tomato sauce bombs (which is fatal to Goblins) and are able to repel the attack. When Mulgarath arrives, the family is force to retreat into the kitchen. They then dump the remaining tomato bombs into the over and setting it to explode. Red Cap only discovers this right before the oven explodes, melting him and all the other Goblins.
Red Cap was very intelligent for a Goblin. He wore clothes, could speak Human language with no apparent difficulties, and he walked on two legs. The books only describe him as "a large Goblin" with teeth made from bottle glass. However, in the film, he was nasty, vicious, grouchy, bossy and slightly arrogant, as he was often seen shouting out orders and pushing the other Goblins around.. He is shown to be vary annoyed with how stupid the other Goblins are, often insulting them.
Red Cap was often seen carrying a cutlass and seemed to be a very skilled swordsman, being able to hold his own against expert fencer Mallory.
In the video game adaption of the film, Red Cap is featured as a boss on two locations. His role is slightly different compared to the film. Rather than getting the Field Guide pages when the children are visiting Lucinda, he succeeds in getting them before his fight with Mallory after decking Jared. The fight is also different from the film, since Jared joined his sister in fighting the goblins. Redcap's sword skill is demonstrated in the fight, with the only way to damage him is knocking his sword out of his hand, then attacking him. He is then fought again when Mallory storms the Goblin hideout (the Quary) to get the stolen Field Guide pages back. He then participates in storming Spiderwick Estate with his Goblin Army, but is melted along with all the other Goblins when the stove erupts.