Chronicle of an actor's life and times, triumphs and failures; Double Bill, by Alec McCowen. New York: Atheneum. $10.95.

Alec McCowen's "Double Bill" is more than a double pleasure. For although the British star focuses on the two most momentous experiences in his distinguished theatrical career, his memoir inevitably becomes a brief chronicle of the actor's life and times, his triumphs and failures.

The first role on this twin bill is Fr. William Rolfe, whose fantastic imagined papal adventures were adapted from Rolfe (Baron Corvo) writings into "Hadrian VII," a London and New York hit in 1968-69. In Hadrian's case, the chief interest of the McCowen recollections lies in the account of the tremendous difficulties, the disappointments and revivals of hope that proceded ultimate triumphs on both sides of the Atlantic.

The second role in "Double Bill" is that of the New Testament writer, Mark, whose gospel Mr. McCowen committed to memory and performed to international acclaim beginning in 1978. The section on "St. Mark's Gospel" is an eloquent and often moving account of what have been the profoundest experiences in a rich and varied professional life. Mr. McCowen leads the reader from early efforts and experiments to tentative tryouts, growing acceptance, and final popular success on stage.

Due to the very nature of the New Testament material and the devoted care of the actor, the achievement carries an inspiration seldom associated with theatrical success. Besides sharing some of his own layman insights, Mr. McCowen reveals how the power of the message of faith overcame occasional performing difficulties. The incidents carry their own conviction.

"Double Bill" abounds in the kind of actor talk that fascinates theater buffs -- the anecdotes, witty observations, and sincere tributes to admired colleagues. As much as any stage memoir I can remember, this one tells what it's really likem to be an actor. Scarcely any aspect of the craft remains unexplored or at least touched on. In effect Alec McCowen has provided a triple bill -- a readable, insightful, and valuable book.