Good question!
There are two things I'd recommend here; the first is the easiest, but the second is the most important. All of this applies to both Text Response and Comparing Texts tasks.
First, really DELVE into the minds of the characters. Let's take
Rear Window as an example: why does Lisa want to be in a relationship with Jeff? What does she see in him? Does she love him unconditionally, and if not, what are her conditions? How does the relationship make her feel? What are her beliefs about herself and other people?
Pretend you're a psychotherapist and the character is sitting back on your couch, ready to tell you their innermost thoughts and fears - what would they say, and what questions would you ask?
This gives you a much deeper understanding about their motivations and desires, which in turn leads to a more sophisticated essay where you can go beyond surface level observations like 'Lisa loves Jeff' or 'Lisa wants Jeff to commit to her'.
However, the characters aren't real people you can interview - they're
constructs! This leads us to the second step: asking WHY DOES THE AUTHOR DO THIS? Every single thing that happens in the text - every word, every line of dialogue, every camera shot - is the product of
authorial intention. In English, we assume absolutely everything is deliberate, and then we explore why an author would have made certain choices in order to comment on their significance in our essays.
(Yes, obviously Hitchcock didn't sit there for hours thinking about what colour the fridge should be in order to make a grand statement about the nature of humanity... but if you start by assuming things are meaningful, you'll have a much easier time discovering and coming up with interesting ideas than a student who sits there and sulks about how 'it's just a movie' or 'who cares if the flowers are red; it doesn't mean anything'
)
I'm not sure if you had your Creative SAC on Rear Window, but the idea behind that assessment task is for you to put yourself in an author's shoes and think about how you would construct meaning through language, plot, setting, symbols, etc. Then, for a Text Response essay, you just have to pick apart these elements in your set text and analyse them.
For example, why would Hitchcock have made Jeff a photographer? What does that tell us about the character, or the themes of observation and voyeurism? How would the film be different if Jeff was a pastry chef, or a ninja? (Don't literally answer that last one, but it can be really helpful to ask 'how would the film be different if X was missing/different?')
So in short, get inside the characters' heads, and then get inside the author's head. Acknowledging the intentional choices the author has made to construct a story and portray people in certain ways is an exclusively upper-range essay characteristic, so if you can do this effectively, it's guaranteed to impress the assessors!
If you're feeling stuck, try reading some online resources or any secondary stuff your teacher has given you. This is no supplement for original ideas, but it can be a great starting point.
Hope this helps!