If Castiel were to be in this crossover, he would be very confused by the walkers at first. He's used to dealing with demons and angels, not undead creatures. But he would quickly adapt and use his angelic powers to protect the group. Rick from The Walking Dead would see Castiel as a valuable asset but also be a bit wary of his otherworldly nature. They would have to learn to trust each other as they both have the goal of protecting the survivors and finding a way to end the chaos in their respective worlds.
The characters from Supernatural would bring their unique skills. Dean, for instance, is an excellent marksman and fighter. He could easily adapt to using guns against the walkers. Sam, with his knowledge of the occult, might be able to figure out if there's a deeper, supernatural reason for the zombie outbreak. Castiel, the angel, could use his powers to heal the injured in Rick's group or even fly over areas to scout for threats. They would add a new dimension to the fight for survival in The Walking Dead world.
Sam and Oliver would probably start off a bit cautious with each other. Sam, being the more cerebral of the Winchesters, would be interested in Oliver's methods of protecting the city. Oliver, on the other hand, might be a bit skeptical of Sam's supernatural knowledge at first. Dean would likely bond with Diggle over their combat skills. They'd probably exchange stories about their battles, and Dean might even show Diggle some of his unique ways of taking down enemies. Castiel, with his angelic powers, would be a mystery to the 'Arrow' characters. Felicity might be both intrigued and a bit scared of him, but she'd also be eager to understand how his powers work.
Mabel would be excited to meet the Winchester brothers. She might try to get Dean to tell her all his hunting stories and would probably try to knit them some cool accessories. She could also introduce Sam and Dean to Waddles. Dipper would be a bit more cautious around them at first, but as they started working together, he would respect their skills and knowledge. The characters would need to learn to trust each other quickly since they come from different worlds but are facing similar threats.
Dean Winchester would likely be charmed by Lorelai Gilmore's sass and sense of humor. He might also be a bit protective of her, given his nature. Sam would probably have in - depth conversations with Rory about books and knowledge. Rory, in turn, would be fascinated by Sam's experiences with the supernatural. And Luke would probably be a bit suspicious of the Winchesters at first, thinking they're up to no good in his town.
The powers of the Supernatural characters, like Sam's psychic abilities and Dean's combat skills, would contrast with the more otherworldly powers of the Good Omens characters. Aziraphale's ability to perform miracles and Crowley's demonic powers would be on a different scale. For example, Sam's psychic visions might be able to predict events that Aziraphale and Crowley are involved in, but they might not fully understand how he gets these visions at first.
In a crossover fanfic, the angels and demons from Supernatural could be involved in The Walking Dead universe. For example, an angel might be sent to Earth to stop the spread of the undead, believing it's part of a grand plan to save humanity. Meanwhile, the demons could be taking advantage of the chaos, making deals with desperate survivors. The Winchester brothers would have to navigate this new dynamic while also dealing with the walkers.
On the other hand, Sam and Dean's knowledge of the supernatural would be invaluable to Supergirl. They could teach her about different types of demons, how to detect them, and what weaknesses they have. Their skills in using various weapons like the Colt or salt - loaded shotguns could be combined with Supergirl's powers. For instance, Dean could shoot a salt - loaded shotgun at a group of demons while Supergirl uses her super breath to blow the salt towards a wider area, maximizing the effect.
Gibbs from NCIS would probably be very skeptical of the Supernatural elements at first. He's a man of facts and evidence. But as he sees things that can't be explained, he might start to open up. Dean Winchester, on the other hand, would likely be impatient with the slow - moving NCIS investigation process. They would need to find a middle ground to interact well.
Dean would probably be very suspicious of Hannibal from the start. He's seen a lot of bad guys, and Hannibal's calm and collected demeanor would set off his alarm bells. Sam, on the other hand, might be more interested in understanding Hannibal's motives. Hannibal would likely see the Winchesters as fascinating specimens. He might try to analyze their relationship and use it to his advantage.
Bill Cipher would be a major wildcard in their interactions. If he were to cross paths with the Winchesters, he would likely try to manipulate them for his own amusement. He might offer them false information or try to pit them against the other characters in Gravity Falls. The Winchesters, being used to dealing with devious creatures, would have to be on their guard and figure out a way to outwit Bill while also trying to solve whatever mystery has brought them to Gravity Falls in the first place.