What is the difference between dissertation and contextual essay?

A thesis is a hypothesis or conjecture.
A PhD dissertation is a lengthy, formal document that argues in defense of a particular thesis. (So many people use the term ``thesis'' to refer to the document that a current dictionary now includes it as the third meaning of ``thesis''). Two important adjectives used to describe a dissertation are ``original'' and ``substantial.'' The research performed to support a thesis must be both, and the dissertation must show it to be so. In particular, a dissertation highlights original contributions. The scientific method means starting with a hypothesis and then collecting evidence to support or deny it.
The contextual analysis essay is a kind of work that disintegrates a piece of writing into small parts, analyzing each one separately. This allows us to point out the true intentions of the author, and analyze the whole context. Almost anything can be a subject of such an analysis – a book, a play, a magazine article, etc., but how to write such an essay? It would be quite an easy task, if you are a pro, but what about the newbies?