Vivien Leigh and Laurence Olivier were both married to other people when they first met. It was during production of "Fire over England" (1937) that the two fell in love and started having an affair. After their divorces came through in 1940, Leigh and Olivier got married; their marriage lasted until 1960 when Olivier left Leigh for actress Joan Plowright. As Leigh suffered from bipolar disorder (having frequent violent outbursts), Olivier couldn't live with her anymore. After her marriage to Olivier was over, Vivien Leigh found a new partner in actor John Merivale but Laurence Olivier remained the love of her life.
Here's a telegram from Vivien Leigh to Laurence Olivier which she sent in August 1943, three years after they got married:
LAURENCE OLIVIER OLD PRESTWICK GERRARDSCROSS BUCKS
O MY DARLING ARE YOU ALRIGHT NO NEWS SINCE YOU LEFT IRELAND DESPERATELY WORRIED YOUR BACK ARM SHOULDER KNEE LIP AND WHAT OH WHAT ELSE CABLE IMMEDIATELY BABA DO STOP I WORSHIP YOU DYING SEE YOU = VIVIEN OLIVIER