Another reaction could be resignation. If Anzu has been through a lot in the fanfiction already, she might just give up for a moment, but then use that time when Dark Mariku thinks she's compliant to plan her escape later on.
Anzu could also be more calculating. Instead of panicking, she might try to reason with Dark Mariku, asking him why he's doing this and trying to find a way to use his own motives against him to get free.
It could also be related to a miscommunication. Dark Mariku misinterprets Anzu's actions or words in the fanfiction and ties her up as a result. This could then be the starting point for a story of them trying to understand each other better and resolve the misunderstanding.
Anzu would probably be scared at first, but then she'd likely start to think of ways to escape. She's a brave character, so she wouldn't just sit there and accept her fate.
Harry might initially be shocked and angry. He would probably struggle against the restraints and demand to know why Snape is doing this. He could yell at Snape, asking for an explanation and protesting his innocence if he believes he has done nothing wrong.
Sakura could also be more composed in her reaction. Given her growth as a character, she might try to reason with Sasuke first. She could say things like 'Sasuke, I know you must have a reason for this. Let's talk about it.' However, if Sasuke doesn't respond or seems hostile, she may start to feel betrayed. She might then use her medical ninjutsu knowledge to try to find a weakness in the snakes or in Sasuke's technique that is keeping her tied up. She could also start thinking about how she can turn the situation around and make Sasuke see reason.
Hermione might initially be furious. She is a strong - willed character, so being tied up against her will would surely make her angry. She would probably start using her wits right away, trying to figure out a way to free herself, whether it's through magic or by tricking Draco.
Ginny might be initially heartbroken. She could lock herself in her room and cry for days. After all, she loves Harry deeply. But then, she might become angry and start to focus on her own life more. For example, she could throw herself into her Quidditch career or her studies at Hogwarts.
Bella may react by trying to understand Edward's reasons. She could be more rational in some fanfictions and instead of just being sad or angry, she starts to investigate. Maybe she discovers that there are forces at work that she can fight against to get Edward back. Or she could decide that she doesn't need him and focus on her own growth and development, becoming a stronger and more independent person without him.
Harry could initially be angry and struggle. He's a brave character, and being tied up against his will would surely make him fight back. He might yell at Draco, demanding to be released and threatening consequences.
Ginny might be heartbroken at first. She could retreat into herself, spending a lot of time alone in her dormitory or in secluded places around Hogwarts. She may cry a lot and avoid socializing with their mutual friends.
She might be initially shocked. After all, she would have trusted her friends a great deal. Then she could become very sad and withdrawn. She might throw herself into her studies even more as a way to cope, trying to distract herself from the pain of losing her friends.