When Spencer Tracy signed with MGM, he was a character actor better known for his problem drinking (and very public extramarital affair with Loretta Young) than for his movie hits. But the studio made him a star, and by the time Katharine Hepburn was looking for a male star who could play a prototypical American male opposite her very idiosyncratic persona, Tracy was the obvious choice. Tracy and Hepburn became one of the most legendary Hollywood couples of the century, on-screen and off, and their partnership helped to canonize both as important stars. But their personal relationship was complicated by his drinking and his relationships with other women -- including his wife.
—You Must Remember This