"Antonin Artaud's name is associated with a fundamental revolt against insincerity, and especially against insincerity in literature, where the written word corresponds to an attitude or a prejudice." -Wallace Fowlie

At the age of 18, Artaud was "briefly" cared for in a mental sanitarium for a disorder, which included self-induced sleepwalking (he was later discharged from the military for this problem). He spent some time in clinics, and published his first book of poetry in the early 1920s.
He spent some time with the surrealists, but severed his ties. Wikipedia states that he was expelled from the surrealists, for two reasons: firstly, he refused to denounce theatre as an art form that catered to the bourgeoisie, and secondly, because he refused to join the French Communist Party, which many of them had.
After witnessing the work of the Balinese theatre at the Colonial Exposition in 1931, he was inspired to write his major oeuvre, Le Théâtre et Son Double.
He spent some time in Mexico - Wikipedia's bibliography includes some information concerning Artaud's study of the Tarahumaran Natives, which included experimentation with peyote. This visit would greatly influence Artaud's perception of Western Civilization, and further fuel is argument for the necessity of Theatre of Cruelty.
Artaud died in 1948.

"Of major importance in the evolution of Artaud's thought was the Colonial Exposition of 1931, where he was able to observe performances of the Balinese theater. These were the immediate origins of his conception of the Theater of Cruelty for which he wrote the First Manifesto in 1932 and the Second Manifesto in 1933." -Wallace Fowlie

Artaud's distaste for dominance of traditional French theatre stemmed from the structure of the plays themselves: "as...art which states a problem at the beginning of a play, and solves it by the end. It is an art form, he says, that presents a character at the beginning and then proceeds to analyze the character during the remainder of the play." He criticized this method heavily, arguing that the theatre was not created to "elucidate a charater and solve a conflict." He continued to argue that at the time, a new form of theatre was beginning to emerge, "one...characterized by freedom, by the surreal, and by mystery."
His "most cherished dream" would be the foundation of a new French teatre that would abandon the artistic spectacle of tradition. He desired theatre to become a communion between the audience and the players: "As in primitive societies it would be a theatre of magic, a mass participation in which the entire culture would find its vitality and its truest expression."
These primitive societies were integral to Artaud's deconstruction of western civilization. His analysis of theatre included a division of humanity in two: "the primitive or pre-logical group and the civilized or logical group. The roots of the real theater are to be found in the first group." The un-logical group were more in tune with the basic urges of the human, including incest, cannibalism, and deicide - these were accepted, and not determined as negatives, as in Western civilization.

"A dramatic presentation should be an act of initiation during which the spectator will be awed and even terrified--and to such a degree that he will lose control of his reason. During that experience of terror or frenzy, instigated by the dramatic action, the spectator will be in a position to understand a new set of truths, superhuman in quality." -Wallace Fowlie

Artaud used spiritual terms to define theatre: the "sacred goal...is to communicate delirium whereby the spectators will experience trances and inspiration. Dramatic art induces as strong a delirium as a plague does." (note: the beginning of The Theatre and Its Double describes in lengthy detail the plague, delirium, and the effect on Europe.)
Theatre in the Round was a technique employed to establish a close connection between the players and the audience. His production of Les Cenci (adaptation of the texts by Shelley and Stendhal) was an experiment in his theories:
"In this production mechanical devices were used to create a visible and audible frenzy: strident and dissonant sound effects, whirling stage sets, the effects of storms by means of light, unusual speech effects."
The failure of Les Cenci established Artaud as a true prophet of his cause. His mental illness was perceived by some as a revolt, and he frequently protested morality and rationality, becoming a "martyr of illogicality."

"To be an actor for Artaud is equivalent to suppressing in oneself traits that make one distinct from other men. It is therefore equivalent to a kind of suicide. The last long internment of Artaud was interpreted by the surrealists as purely arbitrary, as a sign of the persecution that society is constantly imposing on revolutionaries and dissidents. However one interprets the terrifying obsessions of Artaud, they allowed him to see into unusual depths of the human mind, where he claimed the eternal questions on life and death are clearly visible." -Wallace Fowlie

Theatre is not to copy reality, as traditional French theatre, but was another reality where "the principles of life are always just disappearing from beyond our vision" (this is the 'double' in the title of the text). This reality is not comprehensible to Westernized man. Theatre can therefore be used to lead man to his primitive instincts, bewitching them into a trance.

"Artaud wanted to see stage gesticulations elevated to the rank of exorcisms. In keeping with the principal tenets of surrealism, Artaud would claim that art is a real experience that goes far beyond human understanding and attempts to reach a metaphysical truth. The artist is always a man inspired who reveals a new aspect of the world." -Wallace Fowlie

"The Theatre of Cruelty has been created in order to restore to the theatre a passionate and convulsive conception of life, and it is in this sense of violent rigour and extreme condensation of scenic elements that the cruelty on which it is based must be understood. This cruelty, which will be bloody when necessary but not systematically so, can thus be identified with a kind of severe moral purity which is not afraid to pay life the price it must be paid."
– Antonin Artaud, The Theatre of Cruelty, in The Theory of the Modern Stage (ed. Eric Bentley), Penguin, 1968, p.66

There is no difference, in Artaud's mind, between imagination and reality. Thoughts and delusions are very real; if an audience accepts this as reality upon viewing theatre, they are able to understand that what they are seeing is actually real, and not simply theatrics.