I'm going to have to agree with Shakycow on that one, but if your willing to take the chance, then here are some tips :
- Cut up the video. If you just have un-edited footage of your reaction, it's much more likely to get copyright-claimed. Try and make cuts between important parts of the video, and edit out silence or boring bits.
- Have a bit at the beginning of the video saying that you are respecting fair use. Normally a 3-second clip of text that says that the video is protected by fair-use will work.
- Have a bit at the end of the video where you make a short summary of your reaction. This is something that I think reaction channels should do, and it also stops people from thinking all you did was watching a video whilst saying random comments, and then beg for likes and subs, which is sadly what most reaction channels do. Maybe say what parts you didn't like, what you did like, what you found funny, and make rate it out of 10.
- Leaves links towards the content you react to in the description. This is something that most reaction channels should be doing but aren't. If you don't, you are practically stealing someone's content.
That's really all I have to say. Personally, I don't like reaction channels because it is really generic, and requires almost zero effort to make, but if that's what you want to do, go for it. I know you make other content, like movie reviews, so it's fine if you want to supplement your channel with other content such as reactions, especially if you have a tight schedule. Anyways, hope this helped!