Well, Sirius has a wild streak himself, but when it comes to Harry, he's protective. He may first be shocked to see Harry smoking. After that, he would likely take steps to ensure it doesn't happen again. He might even make Harry do extra chores around Grimmauld Place or something similar as a form of punishment, while also educating him about the perils of smoking.
Sirius might confiscate whatever Harry was smoking. He would then sit Harry down and have a serious conversation about the importance of making good choices. He could also remind Harry of the dangerous magic substances that might be out there that are far worse than normal cigarettes and that he should stay away from any form of harmful substances.
Sirius would be deeply affected. He'd see Harry as a sort of surrogate family, and seeing Harry in such a state would make him feel like he failed in protecting him. He'd do everything he could to make Harry feel better, from getting him medical help to just sitting and talking with him for hours about what's wrong.
It could strain their relationship at first. Sirius would be worried and disappointed, and Harry might feel resentful of being caught. But in the long run, it could strengthen their bond as Sirius tries to help Harry get over this bad habit.
He's probably furious. Snape has high standards for student behavior, and smoking is definitely not acceptable in Hogwarts. His face might turn red with anger, and his eyes would narrow as he glares at Harry.
Well, one possible consequence could be detention. Snape is strict and would likely punish Harry for breaking school rules by smoking. He might make Harry clean the Potions classroom or do some other unpleasant chore.
Well, in that situation, Sirius would be shocked and heartbroken. He might first scold Harry for doing something so dangerous to himself. Then, being the impulsive person he can be, Sirius would probably take Harry to see someone he trusts, like Dumbledore or Madam Pomfrey. He would not let Harry out of his sight until he was sure Harry was okay and not likely to do it again.
There is a fanfiction titled 'Under Sirius' Watch'. In it, Sirius catches Harry and Draco sneaking around the castle at night. This leads to a series of events where Sirius has to decide whether to report them or try to understand what's really going on between the two. The story delves deep into the characters' emotions and the complex relationships within the Hogwarts world.
One common theme is the theme of mentorship. Sirius often takes on a mentorship role when he catches them. He uses his knowledge of the wizarding world, especially the darker aspects, to guide Harry and Hermione. Another theme is friendship. Their friendship is tested and strengthened when Sirius catches them in some kind of 'forbidden' activity. It shows how they support each other through difficult situations.
There are also fanfictions where Sirius catches them and instead of being angry, he is curious. Like in 'Sirius' Curiosity', he decides to observe them secretly for a while to figure out what their relationship really is, rather than jumping to any hasty judgments.
In a fanfic scenario, if Remus catches Harry cutting, he would likely be very concerned. Remus is a caring character. He might immediately try to stop Harry and find out what's wrong. He could use his calm and understanding nature to get Harry to open up about his troubles.
He might be initially shocked. But if they were just having a friendly chat or doing something normal, he would quickly recover and probably join in the conversation. Harry respects them both a great deal, so he'd be more curious than anything else.