Does the character in the novel have to be marked with "shuo" or "dao" before speaking?

Before people speak in novels, sometimes the words "shuo" or "dao" can be omitted.

But the premise is that the reader has made it clear about the subject of the speech. Only in this case can the subject of the speech (the subject of the sentence) be omitted, and markers such as "shuo" or "dao" can be omitted. Otherwise, it is easy to produce different meanings.