Past or Past Perfect

It had happened at last. The expected message had come. All his life, it seemed to him, he had been waiting for this to happen.
He was walking down the long corridor at the Ministry and he was almost at the spot where Julia had slipped the note into his hand when he became aware that someone larger than himself was walking just behind him.

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Isn't the past perfect necessary as we are still in the same time frame as the earlier paragraph (It had happened at last. The expected message had come...)?

Not necessarily. The first paragraph tells us that the event occurred, and does so from the point of view of a moment in the past when the event was in the past, but fresh in the mind of the protagonist, Winston. Using past perfect this way heightens the excitement. The second paragraph switches to past continuous to narrate the actual sequence of events.

Blue JayUsing past perfect this way heightens the excitement.

Is the reason the past perfect was used int eh first paragraph? I thought it was no for effect but because it is required, as the event happened prior to the main tense of the novel. (The book is written in past tense).

No, it could all have been written in the past, because the events of this chapter are all part of a sequence of events in the past. Orwell could have written, "The following day Winston was walking down the corridor thinking about Julia when he saw O'Brien..." He chose to switch to past perfect to heighten the tension. Paradoxically, the use of the past perfect grabs the attention of the reader,