David Lean wanted his film to have a feeling of heightened realism. Working closely in conjunction with art director John Bryan and cinematographer Guy Green, he employed several tricks, such as forced perspective, to achieve this effect. The famous opening shot in the graveyard, for instance, features a brooding church in the background which in reality was only 3 meters high.
David Lean was not a particularly well-read man, and only became aware of the power of Charles Dickens' story when his wife Kay Walsh dragged him along to a theatrical production of "Great Expectations" in 1939. Incidentally, playing Herbert Pocket in this production, was a young Alec Guinness, whom Lean subsequently cast in the same role in the film version. Aside from bit parts, it was Guinness' first major screen role and was also the first of six films he made with Lean. Martita Hunt was also in the stage production, playing Miss Havisham, a role she reprised in the film.
Charles Dickens wrote a much harsher ending to the original novel for its original publication; he was forced to substitute the more famous ending when it was published as a book later on, because the original one upset his readers. I believe that this was the only time he ever changed his novel's ending.
The Cineguild team of David Lean, Ronald Neame and Anthony Havelock-Allen collaborated on the screenplay after Dickens' scholar Clemence Dame could not come up with a worthy script. Like Dickens, the Cineguild team was stuck for an ending. In the novel, which actually had two different endings, Dickens had Pip and Estella briefly meet when they are elderly. It was Lean's wife, Kay Walsh, who came up with the idea of Pip rescuing Estella from the fate that mirrored Miss Havisham's. And for that, she received a screenplay credit.