From Chapter 48 of Charles Dickens's Great Expectations:
“Mr. Jaggers was for her [Molly, Mr. Jaggers's maidservant],” pursued Wemmick, with a look full of meaning, “and worked the case in a way quite astonishing. It was a desperate case, and it was comparatively early days with him then, and he worked it to general admiration; in fact, it may almost be said to have made him. He worked it himself at the police-office, day after day for many days, contending against even a committal; and at the trial where he couldn’t work it himself, sat under counsel, and—every one knew—put in all the salt and pepper.
The 1933 OED oddly does not mention 'salt and pepper' but does contain the following entry for
salt, sb., 3.c. That which gives life or pungency to discourse or written composition ; poignancy of expression ; pungent wit.
Mr. Jaggers is depicted as being quick to punish ill-applied usages of words and other such minute discrepancies with noticeable vehemence, so could 'put in all the salt and pepper' refer to his imparting a certain flamboyance to the legal proceedings?