Reading those fics, the tension often comes from people writing Fred as someone who sees past Hermione's reputation. He isn't just the class clown; he's observant. He notices when she's stressed about S.P.E.W. or when she's biting her lip over a Charms text, and he'll deflect with a joke that actually makes her laugh, not feel patronized. That creates this quiet intimacy where Hermione feels understood in a way Ron or even Harry sometimes don't. The romance simmers because it's built on a foundation of mutual respect that the books only hinted at with their brief interactions.
Another classic method is using the war. Post-war fics, or AUs where Fred survives, lean into survivor's guilt and shared trauma. Hermione, who carries so much, finds someone who uses humor as a shield just like she uses books. Their bonding feels less like a whirlwind and more like two people carefully dismantling each other's walls. The tension isn't will-they-won't-they in a high school sense; it's whether they'll allow themselves this happiness after everything. A story I read had them rebuilding the joke shop together, and every shared memory of the Battle of Hogwarts was another thread pulling them closer, loaded with this bittersweet weight.
Physically, writers get clever too. It's never grand gestures. It's Fred brushing flour off her cheek in the Gryffindor common room kitchen, or Hermione fixing the collar of his robes before a Ministry function. Small, charged moments that highlight how comfortable they are in each other's space, which feels earned after chapters of emotional build-up. The best fics make you feel like you're watching two puzzle pieces finally click, with all the friction and relief that entails.