Dickens worked in close collaboration with his illustrators, supplying them with an overall summary of the work at the outset for the cover illustration which was printed on heavy colored stock, usually green, which served as a wrapper for each of the monthly parts. Dickens briefed the illustrator on plans for each month's installment so that work on the two illustrations could begin before he wrote them.
This close working relationship with his illustrators is important to readers of Dickens today. The illustrations give us a glimpse of the characters as Dickens described them to the illustrator and approved when the drawing was finished. Film makers still use the illustrations as a basis for characterization, costume, and set design in the dramatization of Dickens' works.
Popular illustrator who became an early friend of Dickens, illustrating Sketches by Boz and Oliver Twist. Cruikshank also acted in Dickens' amateur theatrical company. Their friendship cooled when Cruikshank, formerly a heavy drinker, became a fanatical teetotaler in opposition to Dickens' views of moderation. Cruikshank later claimed that the idea for Oliver Twist had been his.
An excellent source of information about Dickens illustrators (including the above) can be found here.