Some writers might make Katniss more vulnerable in the 'lemon' fanfictions. In the original story, she is strong and stoic, but here she could show more of her softer side, especially in her relationship with Peeta. For Peeta, he might be portrayed as more assertive in the romantic aspects. Usually, he is the more gentle one, but in these fanfictions, he could take the lead more often.
Well, in many fanfictions, Katniss' character can be seen as more open to her emotions when it comes to the 'lemon' parts. She might not be as guarded as she was in the books. As for Peeta, writers may give him a more complex emotional range. He could be not only the kind - hearted boy but also show a bit of a wilder side when it comes to his relationship with Katniss. This is different from the more one - dimensional view of him in the original story. Also, Katniss might be more physically affectionate, which is a big change from her reserved nature in the Hunger Games series. Peeta, on the other hand, could be more confident in expressing his desires towards Katniss.
Writers often change the way Katniss and Peeta interact. In the 'lemon' Hunger Games fanfictions, Katniss may be less focused on survival and more on her relationship with Peeta. She could be more spontaneous and less calculating. Peeta, instead of just being the sweet, artistic boy, might be shown as a more dominant figure in the relationship at times. This doesn't mean he loses his kind nature, but it adds a new layer to his character. Writers also might make Katniss more in touch with her femininity. She was a tough hunter in the original, but here she can be more of a woman in love, with all the associated emotions and behaviors, while still retaining her strength.