When we joined the studio in 1990 the Thief had been in production in some form for 25 years. The studio had made a lot of money on commercials, which Dick had invested into the feature.
Whenever possible he had his artists working on the Thief in between and sometimes even during commercial assignments. He had worked out a deal with Ken Harris to bring him out of retirement. For several years Ken, who lived in the US, spent 6 months every year working at Dick's studio in London. (This started when Chuck Jones produced the Christmas Carol and brought Ken on to animate at Dick's studio in 1971 or 72.) Ken contributed lots and lots of animation on the Thief character. Since his Warner days he had a personal goal of animating 30 feet every week. Another major animator on the film was Art Babbitt, who animated scenes with King Nod, the Cobbler, the Dying Messenger and Zigzag. Initially Dick had put some of the work through the system. He would clean up the drawings, add overlap etc. It then went through trace and paint and camera. Over the years the designs evolved however, and in order to keep the models consistent Dick found himself redoing scenes that were done in final color already. To avoid this Richard Purdum, one of the directors working at the studio in the 70's, suggested to keep the animation in the line test stage and save it to be reworked later. After Roger Rabbit Dick finally was able to find outside funding to start working on the film full time with a bigger crew. Ken had died in 1982, while Art was still alive, but already in ill health (He died in 1992). Art had also given a lot of lectures at Dick's studio and Dick based a lot of his own Master Class and his book The Animator's Survival Kit on Art's teachings. By 1990 Dick had already finished a lot of scenes with the Thief and had nothing but praise for Ken's animation. He now started to delegate a lot of this work to Neil Boyle and later also to Alan Simpson and myself. I think the Thief was Dick's favorite character, but he also wanted to establish the design and animation style for some of the other characters: The Cobbler, King Nod, Zigzag and Yumyum. He did this by animating the first scenes himself and then picked some of his animation poses to create model sheets.
Sometimes when working over Art's animation he could be heard voicing his frustration out loud. In retrospect, while giving Art a lot of credit for his earlier work and especially appreciating him as a teacher, he realized that Art's work on the Thief had it's problems (one has to consider that Art had been in his seventies) and that he needed to rework it much more than Ken's and often even completely redo scenes. He told us that he also regretted giving in to Art requesting model changes to characters that he had been assigned to animate. He had been Art's employer and director but also his student and at the time, out of respect, had not allowed himself to be critical of his mentor.
To balance things out you might want to look at this: