difference between different forms for stative verbs

The reporters had been astonished by his sudden arrival in town; they had thought that he would not wish to be there just when the worst of the scandal was about to explode on the front pages. But they had been wrong.

The reporters were astonished by his sudden arrival in town; they had thought that he would not wish to be there just when the worst of the scandal was about to explode on the front pages. But they were wrong..
The first one makes further text explicit: the reporters were later no longer astionished, etc.
i'm sorry .... i didn't get you.
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[...had been astonished......no longer astonished...NOW]

[...were astonished...NOW]

Had been astonished gives a little more info on the timeline.
Hello again, gulliano.

The reporters were astonished by his sudden arrival; they had thought that he would not wish to be there. But they were wrong.-- The two underlined verbs are simple past tense; they are just a statement that those actions happened before NOW. But notice the bold verb: the past perfect tense is used to make it clear that the reporters 'thought' before they 'were astonished'. Past perfect is used for an action that happened before another past action, while simple past is used for an action that happened before NOW.

The reporters had been astonished by his sudden arrival; they had thought that he would not wish to be there. But they had been wrong.-- Therefore, when past perfect is used in this sentence, then somewhere in nearby text must be another past action or actions that happened after 'had been astonished' and 'had been wrong'. That is why I said that this second sentence makes further text explicit.
Thanks a lot mr. micawber.
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