Getting close to a meltdown
First time Vivien came dangerously close to a mental breakdown was during the making of Gone with the Wind. Here, it was nothing worrying by itself though - the circumstances under which she was put could turn the most stable individual into a nutcase.
Despite being an underpaid underdog, suddenly Leigh found herself in the spotlight. Media were first destroying her, saying that there is no place for an Irish actress playing a typical American heroine, and later they started hyping her obsessively.
The latter made people's expectations astronomical and Vivien knew that there is now an incredible pressure to produce a star performance. The star focus made Clark Gable and few other people unhappy that they were outshined by the relative unknown. They were all too classy to mention it and held no grudge against her directly, but the dissatisfaction was there.
The stabilizing factor was George Cukor, a director who focused most of his effort on Leigh and Olivia de Havilland. With a friendliness and a near-paternal care coming from him, Vivien was able to sustain the outside pressure, but even that changed. After Gable's complaints Cukor was dismissed and Fleming took his place.
Vivien found his vision completely out of touch with what the book was about, her contact with him was neutral and now Gable was under the director's crosshair. Tough as Fleming was, after few months of soaking the nervous atmosphere on the set, he already wanted out!
The need for improvisation took its toll on all the actors. Scripting was poorly organized, which resulted in lines being written on a day-to-day basis. Vivien was great at memorizing lines, but even for her it was taxing to have to learn them on a short deadline.
To make things worse, there was no chronological order in the shooting, which forced her to jump from one period to another, often in reverse, looking differently and playing differently.
As Vivien herself said, she worked like a mule all day long, and when she was coming to bed completely devoid of energy, most of the time she was finding herself restless, unable to regenerate! And so she either slept few hours, or not at all.
Day after day, she was becoming more zombie-like, which many people on the set confirmed. Olivia de Havilland herself had the easiest time noticing it - after a month's absence from the set, she came back to see a completely different Vivien than the one she saw month ago when leaving. In fact, the difference was so pronounced that she passed her on a corridor without realizing it was Vivien that just went by!
For the first time, Vivien Leigh flirted with a mental breakdown. She didn't cross the line, but claimed she came close.
Even before Gone with the Wind had its premiere, Hitler started World War 2 by teaming up with Russia and beginning the occupation of Poland. Physically, Leigh was shielded from the issue because she was on another continent, but London was where she had spent considerable amount of time, she had many friends there, and most importantly: her family was there.
Before having an opportunity to rest after the exhausting experience of playing in her most popular film, she was instantly thrown into something hundred times more devastating.
She felt the need to be on the spot, doing whatever she can to help and so did the love of her life Laurence Olivier, so as soon as they possibly could, they boarded the ship to England.
Leaving sunny Hollywood and arriving in London destroyed by bombs, seeing familiar buildings torn to pieces (including her home) seems similar to leaving India for convent as a child. It was an absolute shock treatment.
Seeing Olivier so eager to join the military (he wanted to become a pilot in Royal Air Force) gave her another worry - what if he succeeds and then his plane is shot down during a mission?
Despite all that, she remained surprisingly strong and began touring with a group of actors to raise morale. When the tour came to North Africa, conditions which actors had to withstood were so terrible that it wouldn't be surprising if she'd flip at some point.
There is no environment that would trigger all sorts of mental illnesses more than an armed conflict on a grand scale, but surprisingly Vivien got through it with her head held high. Perhaps seeing how everyone else around her suffers in all kinds of ways made her own problems seem trivial in comparison?