Well, being transported into Merlin fanfiction would be like stepping into a whole new realm. The culture would be different, with knights and lords ruling the land. You could find yourself in the middle of a battle between good and evil. You might also discover new magical creatures that were only mentioned in the fanfiction. You could try to use your modern - day knowledge to help the characters, but at the same time, you'd have to adapt to their way of life. For example, you might introduce new strategies for fighting battles or new ways of dealing with magical threats.
In a more humorous take on the 'Merlin tied to a bed' fanfiction, it could be that Merlin has fallen ill and the overly - protective Gaius has tied him to the bed to make sure he rests. Merlin, being the active person he is, complains constantly. Arthur comes to visit and teases Merlin about finally being still for once. Meanwhile, Merlin tries to convince Arthur to let him loose, promising that he'll be fine. Eventually, Gaius relents and unties Merlin, who then immediately starts bustling around the room.
Finn. He's the main hero, so it's very likely for him to be in such a situation. His adventurous spirit makes it a great plot point for him to be transported somewhere new and exciting. Jake is also a likely candidate. His shape - shifting abilities could play an interesting role if he were transported. And the Ice King, because he often gets into strange situations already. His craziness could lead to him being accidentally transported.
If Merlin died in the fanfiction, it could trigger a series of tragic events. The balance of power could tip, and the villains might seize the opportunity to attack. It could also bring out hidden strengths in the remaining characters as they try to cope with the loss.
One more idea is that Merlin is tied to the bed in a dream - like state created by an ancient magic. In this dream world, he is faced with his deepest fears and insecurities. His friends in the real world notice his strange behavior and try to wake him up. Merlin, while in the dream, has to find the source of the magic that is binding him and break free. Through his inner strength and the memories of his friends, he manages to do so and wakes up, having learned more about himself and his magic.
If Merlin is a Dark Lord in fanfiction, it could be an interesting exploration of the 'nature vs. nurture' concept. Maybe he turned dark because of the constant fear of being discovered and persecuted for his magic. His actions as a Dark Lord might include taking over Camelot or exacting revenge on those who wronged him in the past. It gives a whole new perspective on his character.
First, make friends with the friendly characters. They can help you understand the rules of that world. For example, if you befriend Simba in 'The Lion King' world, he can protect you from the hyenas. Second, learn the magic or special skills there. In 'Frozen', learning how to control ice magic could be useful for self - defense.
They can start by observing the new world's culture. Just like when we travel to a new place in our world, the OC can watch how the people interact, what their values are. For example, if the new world values honor above all else, the OC can try to uphold that in their actions.