A child actor on radio by 1936, Rain's study at the Banff Centre for Continuing Education (now the Banff Centre for the Arts) led to a scholarship to the Old Vic School of Theatre in London. Returning to Canada, he performed with CBC and Toronto's Jupiter Theatre, starring in Jean Anouilh's Ring Round The Moon (Royal Alexandra Theatre,
1954). He joined the first Stratford Festival Company in 1953, as Dorset and understudy to Alec Guiness in Richard III. A member of 31 of Stratford's 45 subsequent seasons, he demonstrates
an equal facility with Shakespearean and other classical roles: Malvolio in Twelfth Night, 1957; Prince Hal in Henry IV, parts 1 and 2, 1958, 1965; King John, 1960; Iago in Othello, 1959, 1973; Macbeth, 1978; Shylock in The Merchant of Venice,
1996; Orgon in Molière's Tartuffe, 1968; Mosca in Ben Jonson's Volpone, 1971; Sid in Eugene O'Neill's Ah, Wilderness!, 1990; the Walrus/Humpty Dumpty in Alice Through the Looking Glass, 1994; and Tiresias in Oedipus Rex,
Rain appeared at the Shaw Festival in Arms and the Man, 1967, but more frequently in the 1980s: Caesar in Caesar and Cleopatra, 1983; Andrew Undershaft in Major Barbara, 1987; and the Waiter in You Never Can Tell, 1988. In addition, Rain performed leads in Duet For One (National Arts Centre, 1981); Ibsen's The Master Builder (Tarragon Theatre, 1983); and Jitters (Centre Stage, 1986). Internationally, he returned to London for roles in Peter Luke's Hadrian VII, 1968, and Morris West's The Heretic, 1970, before American appearances in Peter Barnes's The Ruling Class in Washington, DC, 1971, and Robert Bolt's Vivat! Vivat! Regina, 1972, for which he received a Tony nomination.
Called "an actor's actor" for the "silken, elegant perfection" of his performances, between 1974 and 1977 Rain was Head of the English Section of the National Theatre School (see Theatre Education). The clarity of his voice has attracted the admiration both of his peers and a vast audience who do not know his name: Rain was the voice of Hal the Computer in the movies 2001 A Space Odyssey (1968) and its sequel 2010 (1984).