Your topic sentences are really your argument sentences. They are the places in your essay, where your argument (what you are going to prove over the course of your paragraph) is clearly and efficiently stated.
It can be helpful to break down the parts that make a good topic sentence into individual pieces. These can be represented with the acronym C.A.R.S.:
C - Context: A clear reference to something unique to the story or the text that you're writing about (this can be a character's name, a novel title, or the name of a setting). You can tell whether or not your topic sentence has clear enough context if you were to give it to a stranger (who has also read the same story), and they would know which story you were writing about from that one sentence alone.
A - Argumentation: A description of the point you're trying to prove to be true over the course of the paragraph. An argument has two parts:
Opinion - Something that you believe to be true, but that could be reasonably disagreed with
A Pattern of Evidence - A description of a pattern or category of events or moments in the story, which all back up your opinion (NOTE: this shouldn't be a list of events; it should be a single description that could fit several different events)
R - Reasoning: An explanation of the underlying reason(s) why you think the main idea of your argument tends to be true (TIP: you can usually start the reasoning section of a topic sentence with the word "because")
S - Significance: An explanation of why the ideas in your topic sentence are interesting or important, or what central lesson about society or human nature that your reader can gain if they read your paragraph (TIP: with a bit of attention to your phrasing, this can be done in the same phrase as your Reasoning)
Here's an example of a successful topic sentence from a character analysis essay that focuses on George from Of Mice and Men:
George's patience allows him to better support people in need, because patience prevents people from damaging important relationships by making rash decisions.
Context - This is easily accomplished by simply mentioning the name "George." It's worth keeping in mind that at some point, the author will write an introduction paragraph that will mention these facts: George is a character in a novel, the novel is titled Of Mice and Men, and Of Mice and Men was written by John Steinbeck. If this sentence shows up later in the same essay, then it is quite clear that the paragraph will be using examples that relate to George's patience.
Argumentation - This topic sentence makes an argument about patience, (which is the opinion: patience allows people to better support others); the pattern of evidence is implied by the description of George helping people in need (which is something he does several times over the course of the novel).
Reasoning - This is something you can see in the second half of the sentence. It is presented after the word "because." This part of the sentence explains how it is that patience has the effect that it does (allowing people to better support people in need).
Significance - This is explained in the same part of the sentence as the reasoning. By shifting the langauge a little bit so that the language in the second half of the sentence focuses on the concept of patience overall (instead of specifically on George's patience), the sentence is able to make a point about patience in general, rather than George's patience in specific. This allows the reader to gain some insight on the nature of patience, rather than limiting the insight to a fictional character.