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1935–1939: Early career and Laurence Olivier

Leigh's friends suggested she take a minor role as a schoolgirl in the film Things Are Looking Up, which was her film debut, albeit uncredited as an extra. She engaged an agent, John Gliddon, who believed that "Vivian Holman" was not a suitable name for an actress. After rejecting his many suggestions, she took "Vivian Leigh" as her professional name. Gliddon recommended her to Alexander Korda as a possible film actress, but Korda rejected her as lacking potential. She was cast in the play The Mask of Virtue, directed by Sydney Carroll in 1935, and received excellent reviews, followed by interviews and newspaper articles. One such article was from the Daily Express, in which the interviewer noted that "a lightning change came over her face", which was the first public mention of the rapid changes in mood that had become characteristic of her. John Betjeman, future poet laureate, described her as "the essence of English girlhood". Korda attended her opening night performance, admitted his error, and signed her to a film contract. She continued with the play but, when Korda moved it to a larger theatre, Leigh was found to be unable to project her voice adequately or to hold the attention of so large an audience, and the play closed soon after. In the playbill, Carroll had revised the spelling of her first name to "Vivien".

In 1960, Leigh recalled her ambivalence towards her first experience of critical acclaim and sudden fame, commenting that "some critics saw fit to be as foolish as to say that I was a great actress. And I thought, that was a foolish, wicked thing to say, because it put such an onus and such a responsibility onto me, which I simply wasn't able to carry. And it took me years to learn enough to live up to what they said for those first notices. I find it so stupid. I remember the critic very well and have never forgiven him."

In the autumn of 1935 and at Leigh's insistence, John Buckmaster introduced her to Laurence Olivier at the Savoy Grill, where he and his first wife Jill Esmond dined regularly after his performance in Romeo and Juliet. Olivier had seen Leigh in The Mask of Virtue earlier in May and congratulated her on her performance. Olivier and Leigh began an affair while acting as lovers in Fire Over England (1937), while Olivier was still married to Esmond and Leigh to Holman. During this period, Leigh read the Margaret Mitchell novel Gone with the Wind and instructed her American agent to recommend her to David O. Selznick, who was planning a film version. She remarked to a journalist, "I've cast myself as Scarlett O'Hara", and The Observer film critic C. A. Lejeune recalled a conversation of the same period in which Leigh "stunned us all" with the assertion that Olivier "won't play Rhett Butler, but I shall play Scarlett O'Hara. Wait and see."

Despite her relative inexperience, Leigh was chosen to play Ophelia to Olivier's Hamlet in an Old Vic Theatre production staged at Elsinore, Denmark. Olivier later recalled an incident when her mood rapidly changed as she was preparing to go onstage. Without apparent provocation, she began screaming at him before suddenly becoming silent and staring into space. She was able to perform without mishap, and by the following day she had returned to normal with no recollection of the event. It was the first time Olivier witnessed such behaviour from her. They began living together, as their respective spouses had each refused to grant either of them a divorce. Under the moral standards then enforced by the film industry, their relationship had to be kept from public view.

Leigh appeared with Robert Taylor, Lionel Barrymore and Maureen O'Sullivan in A Yank at Oxford (1938), which was the first of her films to receive attention in the United States. During production, she developed a reputation for being difficult and unreasonable, partly because she disliked her secondary role but mainly because her petulant antics seemed to be paying dividends. After dealing with the threat of a lawsuit brought over a frivolous incident, Korda instructed her agent to warn her that her option would not be renewed if her behaviour did not improve. Her next role was in Sidewalks of London, also known as St. Martin's Lane (1938), with Charles Laughton.

Olivier had been attempting to broaden his film career. He was not well known in the United States despite his success in Britain, and earlier attempts to introduce him to American audiences had failed. Offered the role of Heathcliff in Samuel Goldwyn 's production of Wuthering Heights (1939), he travelled to Hollywood, leaving Leigh in London. Goldwyn and the film's director, William Wyler, offered Leigh the secondary role of Isabella, but she refused, preferring the role of Cathy, which went to Merle Oberon.