Look over chapter 39 once more, with special attention to the atmospheric way Dickens sets the scene for the dramatic re-entry of the convict and the revelation of the source of Pip's expectations. Like a theatrical director he orchestrates sights, sounds and lighting to create the utmost tension and foreboding in the reader as well as his protagonist.
For this blog, consider Dickens the writer as much as you do his story. This is the last chapter of part two, and most certainly comprised an installment of the serialized novel. The stakes were high, therefore, and he rose to the occasion. Don't indulge in an orgy of flattery, but comment on what most strikes you about the opening 30 paragraphs or so of the chapter leading up to Pip's realization: diction, tone, pacing, etc.