Harry Potter himself is often a central character. The truth serum might be used on him, or he could be the one trying to find out the truth from others. Hermione Granger is also common, as her intelligence might be used to either create or counteract the effects of the truth serum.
One possible fanfiction could be where Hermione accidentally drinks the truth serum during a potions class experiment gone wrong. She starts revealing all sorts of secrets, like her true feelings about Ron and Harry, and also some hidden knowledge she has about Hogwarts' secrets. This could lead to a series of comical and yet somewhat tense situations as her friends try to keep her from saying too much.
Well, in the 'Harry Potter under truth serum' fanfiction, it's likely that we'd see a lot of confessions. Harry could potentially reveal his innermost fears about Voldemort and his own destiny. Teachers like Snape might spill the beans on their past relationships or grudges. It would be a great opportunity for the author to explore the unspoken thoughts and emotions of the characters in a very direct way.
In some 'Harry Potter truth serum fanfiction', there could be a plot where the truth serum is used by students to play pranks on each other at Hogwarts. But these pranks could quickly turn serious as secrets are spilled that were meant to be kept hidden. This could lead to a rift between friends until they figure out a way to fix the situation.
Probably Draco Malfoy. He's always been a bit of an enigma. With the truth serum, we'd get to know if his mean - spirited actions were all his own or if he was pressured by his family. He might also reveal any hidden admiration he has for the trio or any remorse for his actions throughout the series.
Hermione Granger is also a common character. Given her love for knowledge and her skills in magic, she would likely be researching the best spells and magical theories to make the terraforming successful. She might be the one to find ancient spells that are crucial for the process.
In many expelled Harry Potter fanfictions, the Weasley family as a whole gets involved. Molly Weasley would be worried sick about Harry and would be cooking up plans to get him back. Fred and George might use their prank - making skills to expose the truth behind the expulsion. Ginny, being Harry's love interest in the original series, would be actively involved in trying to clear his name. And don't forget about the Hogwarts professors. McGonagall would probably be torn between following the rules and believing in Harry.
Harry Potter himself is often a central character. He might be shopping for new school supplies at Flourish and Blotts or looking for a new broomstick at Quality Quidditch Supplies. His friends, like Hermione and Ron, usually accompany him. Hermione might be more interested in books and rare magical artifacts, while Ron would be eyeing the latest Quidditch gear.
Mrs. Weasley is a very likely character. She's an expert at cooking and baking in the normal wizarding world, so in fanfiction about magical bakes, she could be teaching the younger characters how to make special bakes. Hagrid might also be involved. He could bring in some really strange ingredients from the Forbidden Forest, like giant acorns, to be used in the bakes.
Hermione Granger is often involved. In some fanfictions, she discovers hidden conspiracies within the Hogwarts library. For example, she might find a book that reveals a plot by some of the older pure - blood families to keep certain magical knowledge from half - bloods and Muggle - borns.
Obviously, Harry, Hermione and Ron are the most likely. They are the main characters and are always together, so it's natural for them to be playing truth or dare. Neville could also be involved as he's their friend. And Ginny might join in too, especially as she gets older and becomes closer to the group.