It is true that we can term "Joseph Andrews" as a 'comic epic poem in prose' because it has almost all the prerequisites that are important for labeling it as a 'comic epic poem in prose'.
Fielding himself termed it as a 'comic epic poem in prose' in the "Preface to Joseph Andrews". Fielding claimed that he was founding a new genre of writing but this was not entirely accurate. There was a long tradition of such writing before him, though it was not completely developed or established. According to Aristotle, Homer had produced a 'comic epic in verse' but again according to Aristotle verse is not the only criterion for poetry. Fielding has only combined the ideal of 'comic epic' and the 'prose epic' to produce what he termed as 'comic epic poem in prose'.
An epic is a story of "a conspicuous man who falls from prosperity to adversity because of his some error of judgment i.e. Hamartia. His death is, however, not essential. But his fall arises a sense of pity and fear in us". It also has heroic style and bombastic language. And a comic epic is just reverse to it in most of its prerequisites except a few.
A heroic epic has a conspicuous hero, grand theme, a continuous action, a journey to underworld, wars, digressions, discovery, high seriousness, a high moral lesson and bombastic diction in it and in "Joseph Andrews" there is an ordinary hero, a journey from one place to another place, mock-wars, digressions, discovery, humour, a high moral and a bombastic diction in it. So, it can be termed as a 'comic epic poem in prose'. We can also call "Joseph Andrews" as "The Odyssey on the road" because both the works, Homer's "Odyssey" and Fielding's "Joseph Andrew" in the first place involve a journey. Like Odysseus, Joseph Andrews after the displeasure of a lady, who is superior from him in position and power, sets out on his way home and meets with many misfortunes on the way by the lady who has fallen in love with him. So it would be fairly justified to call "Joseph Andrews" an "Odyssey on the road". Hence it is a 'comic epic poem in prose' as well.
Unlike a heroic epic, the hero of "Joseph Andrews" is an ordinary boy. He is a foot-man of Lady Booby who has fallen in love with him. But Joseph is very innocent and virtuous. Therefore, he leaves the service of the Lady and goes to meet his beloved Fanny. On the way he has to face many hardships.
Though the action of the novel is not as great as the action of an epic yet it is enough to term the novel a comic epic. Joseph sets out from London to Somersetshire to see Fanny. On the way, Joseph crosses many roads, highways, country sides, stays at many inns and meets many people; all this constitute a big action.
Through the journey of Joseph, Fielding satirizes the society of the day and ridicules them. The corrupt and hypocritical clergy, Parson Trilluber and Parson Barnabas, individual like Lady Booby and Mrs. Slipslop, the Squire of Fools and the Squire of False Promises have been satirized.
The element of wars is very important in an epic and it is no exception in "Joseph Andrews". We see a war took place in an inn where Joseph was insulted by the host. Parson Adams was annoyed and challenged the host. There started the first war between both the parties. Soon Mrs. Slipslop and landlady also joined in the battle. There are many other epical elements in the novel to call it a comic epic.
Another epic convention is the use of digression. There are two major digressions in "Joseph Andrews". There are, seemingly, irrelevant stories of Leonara and Mr. Wilson. Epic writers considered them as embellishments. Fielding, however, makes the interpolations thematically relevant. For, these are not irrelevant in reality.
The formula of discovery, as described by Aristotle, an essential element of an epic, has also been used by Fielding. In the end of the novel, we see that Joseph is recognized to be Mr. Wilson's child and Fanny as the sister of virtuous Pamela.
High seriousness is an important element in epic. But in "Joseph Andrews" there is a great deal of comedy and humour, because it is a comic epic novel. But behind this comedy, there lies a serious purpose of reformation. We have a gamut of vain and hypocritical characters in Parson Trilluber, Parson Barnabas, passengers in the stage-coach, Mr. Tow-wouse, Mrs. Slipslop, Peter Pounce and the various Squires. The surgeon and the lawyer and the magistrate are also some other example of hypocrisy and vanity. Each of these characters provides a great deal of humour and amusement under a serious purpose.
Every epic has a moral lesson in it and this is no exception with a comic epic. Fielding's views on morality are practical, full of common sense and tolerance, liberal, flexible and more realistic. These are devoid of prudish and rigid codes. Fielding wanted to tear the veil of vanity and hypocrisy.
The use of grand, bombastic and elevated language is an important element in an epic. It has heroic diction. But in "Joseph Andres" we see that Fielding has used prose for poetry because it brings us close to the real and actual life and it is much more suitable for Fielding's purpose of dealing with human nature. However, his use of prose is very good, up to the mark and apt for his novel.
So, we can conclude that the theory of the 'comic epic poem in prose' as described by Fielding in the preface of "Joseph Andrews" manifests itself in the novel. Fielding has assimilate the rules and adapted them to his way of writing so well that we are not consciously aware of the formal principles which give unity to his materials. According to Thornbury, "Joseph Andrews" by Fielding is:
"An art which conceals art, but is the art of a conscious artist."
It is true that in "Joseph Andrews", the scale is not as large as one can except in an epic, though it has all other elements of a 'comic epic poem in prose', as claimed by Fielding.