Writing a text response for a film is not too different from writing any other text response.
You have your understanding of the themes, characters etc. and your interpretation of the text as a whole. You have an essay topic, and you'll have a series of arguments answering that particular topic.
Where film is slightly different is the evidence that you'd probably be naturally reaching towards, and also the metalanguage you use. There's a lot of factors that contribute to our understanding of the characters, themes etc. and this is the evidence we'd be using. In this case, these are some of the film techniques.
On how you would pick which one to use, well it's the same as how you would pick evidence from a book. Which one is best for supporting your argument, what you're trying to say. Often you'll be able to tie in a few film techniques together into a couple of sentences
(you could probably criticise this, but just as an example of one way you can do things anyway):
"From the first moments of the film, Kazan cuts to a medium shot to direct the viewer’s attention to the costuming of the “mob” leaving the shack. It is the clothing worn by each man that is representative of their inner conscience."
Then continuing on with the rest of the paragraph, discussing the clothing a bit deeper, what Friendly is like (with quotes from him), then contrast that with Terry and what he's like etc. There's certainly other things in that particular shot you could talk about, but it's unnecessary if it's irrelevant to your argument.