In some fanfics, Arthur's heavier build might be shown through his broader shoulders and more muscular physique compared to Merlin's slighter frame. This can be used to symbolize Arthur's strength and power.
Yes, in some fanfics Arthur is depicted as heavier. It could be part of a creative interpretation where the author wants to give Arthur a more physically imposing presence compared to Merlin, perhaps to emphasize his role as a warrior or leader in the fan - created story.
It can be shown through his actions too. Maybe Merlin is a bit more snappy or short - tempered with Arthur when he's feeling jealous. Like when Arthur asks Merlin to do a task, Merlin might do it with a bit of an attitude instead of his usual eagerness, because he's jealous of something Arthur has or has done.
In such fanfictions, Merlin is often portrayed as a very powerful sorceress. She has an air of mystery around her. Her magic is usually depicted as extremely potent and she uses it to protect Arthur or further their shared goals. She might be shown as a manipulator at times, using her intelligence and magic to influence events in their favor.
Often, the relationship is one of mentorship. A female Merlin in fiction might use her wits and magic to help Arthur make wise decisions. She may be more in tune with the subtleties of the court and the people, and thus be able to advise Arthur better. For instance, she could sense the unrest among the common folk before it becomes a big problem and help Arthur take preventive measures, all while having a more nurturing aspect to her mentorship compared to the traditional male Merlin.
Often in fanfiction, Ygraine is a complex character. She may be portrayed as a victim of Uther's desires in one aspect, but also as a strong - willed woman. When it comes to her relationship with Merlin and Arthur, she could be the one who unknowingly weaves their fates together. For instance, her death could be the starting point of a chain of events that bring Merlin and Arthur closer, and fanfiction writers can play with this concept in many different ways.
In some fanfics, Merlin's hidden love is shown by his internal monologues. He'll constantly battle with himself, thinking about how he can't have this person but still yearning for them deeply. This is a very common way to build up the tension and angst in the story.
Merlin would likely be seen as a bit of an enigma at Hogwarts. His magic is different from the norm, so he might be a bit of an outsider at first. But his kind heart and loyalty would soon win over his classmates. Arthur would be the brave and headstrong character, perhaps a bit like a Gryffindor version of his usual self. He'd be eager to learn about combat magic and would probably be a natural leader in group projects or Quidditch teams.
One possibility could be that Arthur's friends are initially skeptical of Merlin. They might have heard stories about him but meeting in person changes their perception. Maybe they see his loyalty to Arthur and his kind heart, and they gradually become his friends too.
In some versions of this type of fanfiction, Arthur's reaction is complex. He might be stunned into silence when he first finds out. His mind would be racing with thoughts about all the times Merlin has used his magic to save him or Camelot without his knowledge. After the shock subsides, he may have a heart - to - heart with Merlin. They would talk about the implications of the secret, and Arthur would likely start to view Merlin in a whole new light, as not just his servant but as a powerful ally with a great burden.
The physical closeness of Merlin sitting on Arthur's lap can be the start of a deeper emotional connection. Arthur might become more aware of Merlin's vulnerability, which could lead to him being more open and caring towards Merlin in general. For example, he could start sharing his own insecurities with Merlin.