Is this sentence correct?

But they were probably not crucial for the story - without them, the main characters' madness surely would have appeared in some other way.

There needs to be a period after "story" and a capital W on "without."

If there's only one main character, then the word is "character's" (singular possessive).

Yes, the sentence you provided is grammatically correct. It is a complex sentence that expresses the idea that the mentioned elements were not crucial for the story because, even without them, the main characters' madness would have been portrayed in an alternative manner.

To confirm the correctness of a sentence, you can follow these steps:
1. Identify the subject and verb: In this case, "they" is the subject, and "were" is the verb.
2. Check for subject-verb agreement: Both the subject and the verb are plural and agree with each other, so they are in agreement.
3. Analyze the rest of the sentence: The rest of the sentence is structured accurately, using appropriate prepositions and conjunctions.

By following these steps, you can verify the correctness of the sentence and ensure it is grammatically sound.