One of the interesting features of the internet is the rise of various genres specifically adapted to various internet media, and reaction videos are a particularly interesting example. I've been watching a lot of reaction videos recently, at least as background, and have been thinking about the features of this particular genre, and especially the subgenres of series and movie reactions. (There are also music reactions, YouTube reactions, food reactions, and even reaction video reactions, but, besides the fact that I find them much less interesting, most of what I say carries over to them.)
Reaction videos are specifically concerned, as you might expect from the name, with showing reactions. This marks their primary difference, as far as structure and content goes, from their closest cousins, reviews. Reactions and reviews overlap in the things they cover, but they do so for very different ends. Reviews are for (in principle) analysis and assessment so that people can decide whether something is worth seeing (or compare their own assessment of what they have seen to someone else's) while reactions are for (in principle) immediate response; I say 'in principle' because both sides in practice are somewhat messier than that makes it sound.
Because reviews are for assessment, reviews tend to be coy about precise details of what is being reviewed -- you don't want to spoil it for those in you audience who haven't seen it -- and the commentary in a review is usually more overarching. Reactions are in some ways the opposite. Reactors are constantly falling afoul of copyright issues, far more than reviewers, because reactions show as much of the object of reaction as the reactors can get away with. If you are reviewing a movie, people who are watching it don't usually need to know anything about the movie at all. If you are reacting to a movie, however, people who are watching it need to know what you are reacting to at all times, in order to make sense of your reaction. Reactors try to get around copyright limitations in various ways -- they will cut out anything that isn't necessary for the reaction, add watermarks, occasionally alternate video and audio, distort sound and blur video, and the like. One of the secondary reasons why people watch reaction videos is an accidental byproduct of this -- if you feel like refreshing your memory of a movie or television series but don't have the time actually to sit down and watch the original, cut-down versions that you get in reaction videos will often be less than two-thirds of the length of the originals, sometimes considerably less.
Older reactions often seem to have more review-like elements, and you still find occasional cases of reactions blended with review. In practice, though, this has faded due to matters of monetization. A lot of reactors do it to sustain a movie-watching or series-watching hobby; there is ad revenue on video websites, and a lot of reactors also have a system set up through Patreon for people to pay to have the right to suggest and vote on what future reactions will cover. This turned out to be more successful than people originally suspected (much more demand for it than one could have predicted), so some reactors started just doing it for the revenue stream. Reviewers usually review most things that come their way in whatever genres they prefer to review, but reactors are often reacting to things that other people have chosen. And the key point here is that people rarely pay money to have someone watch something and trash it. They may occasionally choose something mischievous, in the hopes of a good reaction, but by and large people pay to see others watch things that they themselves enjoy. Further, people don't click on a reaction video in YouTube in order to watch someone hating on something; if you are doing a reaction series to an anime, the people who are going to watch it are anime fans. To be a reactor you have to be the kind of person who likes most things, because the people who actually watch you are usually people who love whatever it is to which you are reacting.
There is a particular kind of parasocial sharing element to reaction that doesn't exist with reviews; people are watching with the reactor. FTW (First Time Watching) reactions are particularly central to the reaction ecosystem, and precisely the draw of FTWs is to get the fresh first reaction to something. In the streaming era we only rarely get this in real life. I think one of the things that reactions show us is that people are starved for it; part of what we enjoy about art is how other people react to it.
In addition, other people's reactions sometimes highlight things that we don't notice, or give us a different perspective on them. For instance, I've recently been semi-watching (as background while I do other things) Lord of the Rings reactions, and it has made me appreciate that some things in Jackson's The Return of the King that put me off because they make no internal sense (like Gandalf beating Denethor down without any reaction from Denethor's guards) actually play well cinematically (Jackson has built up the audience's annoyance with Denethor so much that people who are watching the movie just as a movie don't care about the plausibility -- Gandalf in that moment is their representative, and they are relieved that, finally, someone is doing it), whereas other things that put me off for story reasons (like Jackson having Faramir try to take Frodo to Gondor) also sometimes baffle people who know nothing at all about the story beyond what the movie itself is telling them.
To take a different example, I once watched a reaction to 2005's The Lion, the Witch and the Wardrobe in which the reactors were from India; they did not know what a beaver was, and did not even know that the movie was based on a book. It was interesting to see them puzzling about this warrior Aslan, and then enthusiastically enjoying the reveal of who he was -- because it was completely unexpected to them, but they did know that the lion is king of the beasts, so it made perfect sense once revealed. It was also quite interesting to see how alarmed they were at Edmund taking Turkish Delight -- people from cultures all over the world can immediately recognize the stupidity of taking food from strange women with magical powers -- and their heightening exasperation when Edmund gives his siblings away yet again. Not having a Christian background, they were a bit baffled by the Stone Table and its aftermath -- but also took it mostly in stride, as something strange and unexpected that nonetheless fit the story. Almost all of the war side of the story, however, made complete sense to them; they recognized the conventions immediately as fitting the kinds of stories they already knew. Seeing the movie in light of an audience that had a very different background highlighted very different aspects of the movie.
C. S. Peirce held that every sign has three constituents: the sign-vehicle or representamen (what we often call the 'sign'), the object, and the interpretant. The representamen links up the interpretant and the object; the interpretant is that which the sign-vehicle connects to the object. For instance, if you see a stop sign, the physical sign (representamen) determines your sensory and mental experience (interpretant) in such a way as to connect it to the object (the practical action of stopping). Reactions show in the case of movies, which are signs, how a change of interpretant (in this case the audience) can change how the movie functions as a sign. This is sometimes -- certainly not always, but sometimes -- enlightening, and I think is, after the sympathetic/parasocial aspect of the reaction a major component in what attracts people to reactions. When we like something, we want to know it better, and when see other people's reactions to a movie, we feel like we understand it better, and sometimes do understand it better.
