Complex sentence: During the show's run, Lansbury developed a friendship with her co-star Joan Plowright, as well as with Plowright's lover Laurence Olivier; it was from Lansbury's rented flat on East 97th Street that Plowright and Olivier eloped to be married.
Split into simpler sentences: During the show's run, Lansbury developed a friendship with her co-star Joan Plowright, as well as with Plowright's lover and future husband, Laurence Olivier. It was from Lansbury's rented flat on East 97th Street that Plowright and Olivier eloped to be married.