In Great Expectations, why does Wemmick give advice at Walworth that contradicts what he has said on Gerrard Street?

The character of John Wemmick is recognized by critics as
one of Charles Dickens's most successful split characters.  Some even term him Dickens's
"most modern man."  For, as he tells Pip in Chapter
25,

The office
is one thing, and private life is another. When I go into the office, I leave the Castle
behind me.

So, when Wemmick
speaks to Pip at Walworth, he talks to him as a friend.  But, when he talks with him in
Mr. Jaggers's office, Wemmick is the clerk of Mr. Jaggers and does not overstep his
position or even indicate that he has any personality or warmth.  Much like Mr. Lorry of
A Tale of Two Cities, he is strictly "a man of business" in
London.

Perhaps, by protraying Wemmick as such a totally
different person at Walworth, which is outside London, Dickens wishes to underscore
Pip's first impression of the city as "ugly, crooked, narrow, and dirty" (Chapter
19).