Does this remind you of someone we know?
The painting above is "The Adoration of the Magi". It remained unfinished because Leonardo left Florence and moved to Milan, though we do not know why he did so. With this painting Leonardo declared his independence from Verrocchio, emerging with a fresh, personal style. Although unfinished, this painting is far more innovative than his previous works.
Although Leonardo produced a relatively small number of paintings, many of which remained unfinished, he was nevertheless an extraordinarily innovative and influential artist.
Because none of Leonardo's sculptural projects was brought to completion, his approach to three-dimensional art can only be judged from his drawings. The same strictures apply to his architecture; none of his building projects was actually carried out as he devised them.
Unfortunately, just as he frequently failed to bring to conclusion artistic projects, he never completed his planned treatises on a variety of scientific subjects.
The Last Supper was quite slow in evolving despite the urgings of Ludovico Sforza and the prior. It actually took Leonardo about four years (1494-1498) What Leonardo required was the utmost freedom in order to correct, modify and achieve special color effects. Moreover, the fresco technique was irreconcilable with his bizarre temperament that led him to alternate periods of intense activity with others of total rest.
Leonardo was notoriously unreliable at completing commissioned works. He would devote months to the problems of content and composition but quickly tired of the physical act of carrying out the painting itself.
The most important of his own paintings during the early Milan period was The Virgin of the Rocks; he worked on the composition for a long time, as was his custom, seemingly unwilling to finish what he had begun.
Leonardo went on a trip and left the painting unfinished. When he returned he found that the paint had run and he never finished the painting.
Alas, Leonardo's interests were so broad, and he was so often compelled by new subjects, that he usually failed to finish what he started. This lack of "stick-to-it-ness" resulted in his completing only about six works in these 17 years, including "The Last Supper" and "The Virgin on the Rocks," and he left dozens of paintings and projects unfinished or unrealized.
The above are various quotes from different sources regarding Leonardo; I believe every statement, after changing the historical details, could also apply to Welles; it seems Orson is in great company, and that maybe there's a reason for that (although I'm sure he would cringe at the comparison).