A magical item could be the link. Let's say a powerful amulet in 'Supernatural' has the ability to connect to a similar artifact in the 'Grimm' world. This causes the characters to be drawn together. Sam could use his research skills to understand the connection while Nick uses his Grimm senses to protect them from any immediate danger. They would then exchange knowledge about their respective worlds and how to fight the threats within them.
The characters can interact through a cross - dimensional rift. For example, Dean Winchester might stumble into the 'Grimm' world and be mistaken for a Wesen at first. Nick Burkhardt would then have to figure out who he really is and they could start working together against common threats.
Writers can start by researching the lore of both shows thoroughly. They need to understand the rules of the 'Supernatural' world, like how angels and demons operate, and also the nature of the Wesen in 'Grimm'. Then, they can find common elements to build on. For example, both shows deal with creatures that are hidden from the normal human world. Writers can use this shared concept to create a scenario where the characters from both shows meet. Maybe a Wesen is causing trouble that has 'Supernatural' implications, so the 'Supernatural' characters get called in, and the 'Grimm' characters are already on the case.
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.
The characters from Resh umbrella might be initially in awe of the superpowers of the Avengers. For example, Iron Man's high - tech suit would be a wonder to them. Meanwhile, the Avengers might be a bit cautious of the Resh umbrella characters as they come from a more shadowy and secretive organization. But they would soon realize that they have to work together and start sharing their skills and knowledge.
Often, the FBI agents from Criminal Minds are initially skeptical of the supernatural elements that the Supernatural characters deal with. For example, Hotch might be very hesitant to believe in the existence of ghosts or demons at first. But as the story progresses, they start to see the evidence and have to work together with the Winchester brothers.
The characters might initially be cautious of each other. For example, Shepard from Mass Effect could be skeptical of the Star Trek crew's non - violent approach at first. But as they get to know each other, they could start to share their skills. The Star Trek crew could teach Shepard about their advanced medical techniques, while Shepard could show them some combat maneuvers.
In terms of the slash relationships, a wizard from HP might fall for an elf or a human from LOTR. Their different magical abilities and cultures would create an interesting dynamic. The demon, if it has a more benevolent nature, could act as a mediator between the different groups. It could use its knowledge of different planes of existence to help the characters understand each other better. For instance, it could explain to the wizards how the magic in Middle - earth is different but still related to their own magic in some ways.
Well, the characters from Familiar of Zero are often from a medieval - like magical world. When they interact with the characters from Halo, which is a high - tech sci - fi universe, there can be a lot of culture shock. For instance, Saito from Familiar of Zero might be completely baffled by the advanced weaponry of the Halo characters at first. But then he could start to learn and adapt.
In some crossovers, the interaction is quite complex. Harry Potter could use his magic to help the Stargate team in combat situations on other planets. Meanwhile, Teal'c might teach the Hogwarts students some self - defense techniques that are more physical in nature. The cultural differences between the two groups also add to the fun. The military - like discipline of the Stargate team contrasts with the more academic and magical environment of Hogwarts, leading to some interesting exchanges.
The technology from the Transformers universe would likely have a huge impact on Remnant. The RWBY characters would be amazed by the Autobots' ability to transform and their powerful weapons. The Autobots, in turn, might be intrigued by the concept of Aura and Semblances.