To make their interactions believable, start with the basic differences in their cultures. Demigods are used to a more physical, hands - on combat style, while wizards rely on magic. So when they meet, there could be some misunderstandings and even a bit of friendly rivalry. Maybe Harry could show Percy some basic spells, and Percy could show Harry how to fight with a sword in a more effective way. Also, they could bond over their shared experiences of having to deal with a lot of responsibility at a young age. Their different powers could complement each other. For instance, demigods could use their physical strength to get into places where magic might not work, and wizards could use magic to solve problems that demigods' powers can't handle.
The characters could interact in various ways. For instance, Percy might be a bit in awe of Thor's lightning powers at first but then realize that his own water powers can also be extremely powerful in different ways. The Avengers might be intrigued by the concept of Greek gods being real in the Percy Jackson universe. They could team up to fight a common enemy that somehow threatens both their worlds. Maybe a villain who wants to use the power of the Greek gods and also the Infinity Stones. In such a scenario, the combined strength of both groups would be necessary to save the day.
Percy would probably be a bit of a wild card to the Order of the Phoenix members. He's used to a different kind of magic and a different set of rules. Harry might be initially suspicious of Percy but also intrigued by his powers. They could start off with a bit of a rivalry as they both try to prove themselves in different ways.
Let the characters grow and change through their interactions. If Harry and Percy become friends, Harry could learn about Percy's world and become more open - minded. Percy could also be influenced by Harry's sense of justice. This development in their relationship would make their interactions more engaging and believable.
When writing believable characters in a crossover fanfic, think about the goals and motivations of each character. In the case of 'Nicky Ricky Dicky and Dawn', they might be motivated by family loyalty and adventure. The characters from the other universe could have different motives. Maybe they're after power or knowledge. Show how these different motives interact when the characters meet. Also, give each character a chance to grow and change. The 'Nicky Ricky Dicky and Dawn' characters can learn from the others and vice versa, which will make them more believable in the new context.
For characters from the Harry Potter universe like Harry himself, remember his key traits such as his loyalty, courage, and his sense of justice. When bringing in characters from Fantastic Beasts, study Newt's shyness yet determination when it comes to his beasts. Make sure their actions are in line with their established personalities.
For Fallout characters in a Skyrim setting, you need to show their confusion and adaptation. They're used to a post - apocalyptic world with high - tech remnants. So when they see Skyrim's magic and medieval - style society, they should react realistically. For example, a Fallout character might initially think a spellcaster is some kind of mutant with strange powers. As for Skyrim characters in Fallout, they'd be shocked by the desolation. They should show their values of honor and courage in a different context, like helping a group of wastelanders against raiders.
In a fanfic, Percy could have a mental battle with Gaea where the lemon plays a key role. Imagine Percy enters Gaea's mind - realm. There, he finds a withered lemon tree. By using his willpower and connection to the ocean (as water is also related to life like the lemon), he tries to revive the tree. As he does this, Gaea is forced to watch and is affected by his actions. If he succeeds in making the tree bloom again, it could signify his victory over Gaea's destructive plans.
The character can interact through friendship. They might bond with Percy over shared interests or experiences. For example, if they have knowledge of modern pop culture from their past life, they could introduce it to Percy and have a good laugh. Another way is through rivalry. They could compete with Annabeth in terms of knowledge or strategic thinking.
The Voltron characters, being used to a more high - tech and space - faring environment, would be initially shocked by the magic in the Percy Jackson world. Keith might be a bit standoffish at first, but Percy's leadership skills could earn his respect. Lance would probably be fascinated by all the different mythological creatures and would try to flirt with some of the female demigods. Hunk would bond with the demigods over food, as there could be some interesting magical foods in the Percy Jackson world that he would love to try.
They could interact in a way where Percy challenges Persephone's traditional views. For example, Persephone might be used to the strict rules of the Underworld and her role as Hades' wife. Percy, with his more modern and free - thinking ways, could make her question some of those things. This could lead to some interesting conversations and a shift in their relationship.