The curtains part to reveal a man in bed. The man is wearing a peppermint-candy-striped nightgown and nightcap. He pounds the pillow. Literally. He sleeps. The curtains close. They open again. The man reappears, wearing a tailcoat and top hat over his night clothes, but he has only one shoe. The man is dreaming.
But Bill Irwin is not dreaming. Instead, Mr. Irwin is launching 80 minutes of nonstop, free-form top-flight clowning. The resulting mosaic, an established success at the American Place Theater here, is called ''The Regard of Flight.'' The full meaning of the title may remain speculative. But at one point, from a perch above the audience, Mr. Irwin contemplates flying, announces that he will do so, and then thinks better of it.
Besides the prefatory bed, Mr. Irwin's principal prop is an all-purpose theatrical trunk. Never was a trunk more imaginatively packed or more wonderfully versatile. It is a clothes closet; a repository for stuffing the Irwin Clown and his nemesis into; and even an interior staircase down which the nimble contortionist gradually disappears. The stairs bit highlights a consummately precise kind of pandemonium, timed and choreographed down to the last chase, sight gag, and pratfall.
Mr. Irwin could rest comfortably on his laurels as a master of acrobatic and eccentric tomfoolery. But he goes further. He delights in spoofing theatrical fads and fashions. Doug Skinner, a ubiquitous, sepulchral-voiced stage manager and accompanist, introduces certain segments with terse announcements: ''Places for the free-association segment . . . the dance segment . . . the kinetic-image segment.'' We learn from him how ''the onstage costume change demystifies the theatrical experience.'' We are told of ''the decline of the playwright and the rise of the actor as poet.''
Passing comments - delivered deadpan and hilariously interpreted - concern the ''new'' theater vs. the proscenium stage and ''the obsessions of the bourgeois theater.'' When interpreting Shakespeare, a modish director pontificates that ''the myth is found in the text.'' For anyone who has ever endured the droning of experts at a theatrical symposium, ''The Regard of Flight'' will prove a thing of irreverent joy.
But no special expertise is needed to relish Mr. Irwin, Mr. Skinner, and (at the performance I attended) substitute second banana Michael McCormick. Besides playing piano, electric organ, ukelele, penny whistle, and glockenspiel, Mr. Skinner sings a couple of wistful ballads.
In sum, ''The Regard of Flight'' is a work of original comic artistry which stars a delightfully zany thinking man's clown. Mr. Irwin comes to his current engagement via a variegated career. Born in Santa Monica, Calif., in 1951, the son of an aerospace engineer and a schoolteacher-homemaker, he grew up on the West Coast and in Tulsa, Okla. He studied theater at the University of California at Los Angeles and the California Institute of the Arts, where he worked with Herbert Blau.
In 1972, Blau and several of his students, including Irwin, moved to the Theater Arts Department at Oberlin College, Ohio. Oberlin was also the home of the Oberlin Dance Collective, with which Mr. Irwin has appeared as guest artist. It was there that he met his wife, the dancer Kimi Okada. They established their home in San Francisco.
After two years at Oberlin, Mr. Irwin enrolled in the Ringling Bros. and Barnum & Bailey Circus School in Florida, where he studied clowning with the legendary Lou Jacobs. Back in San Francisco, he was Willy the Clown in the Pickle Family Circus and appeared with Pickle family performers in Robert Altman's film ''Popeye.''
With collaborators Skinner and Michael O'Connor, Mr. Irwin has created a variety of entertainments inspired by popular American entertainment traditions and combining the arts of mime, dance, avant-garde forms, and clowning. Their pieces bear titles such as ''Still Not Quite,'' ''Murdoch in Some Regard,'' ''Peniel,'' ''Circa/Rebellion,'' ''Unnumbered,'' ''Harlequin Lost,'' and ''Harlequin Found.'' Irwin and Skinner have toured extensively in the United States and abroad. Everywhere is home for the Irwin Everyman.
The Irwin inspirations extend from commedia dell'arte to the great clowns of the silent films, especially Buster Keaton and Charlie Chaplin. To one San Francisco critic, Irwin's dancing suggested Twyla Tharp choreographing for Keaton.
Although Irwin's sketches and pieces are collaborations, he has described himself as ''the main cogitator and compiler.'' Often, a routine will begin with his working in front of a mirror on some abstract problem like shrinking or being squashed. One of the repeated problems in ''The Regard of Flight'' occurs when the Clown as movable object is pulled into the wings by the irresistible suction force of an invisible vacuum. Elsewhere he emulates the Incredible Shrinking Man.
Whatever the inspiration and however he may be described, there is no pigeonholing Bill Irwin. He defies category. His art is the perfect marriage of mixed - or merged - performing metaphors and disciplines. Like all great entertainers, he is unique and inimitable.