According to Richard Burton in a 1977 interview, he bumped into Lee Marvin at a party, and the host said, "Of course, you two know each other". Both were very heavy drinkers during filming, and neither had any memory of working together.
Shortly after filming ended, Richard Burton was hospitalized for alcoholism. At the time, he was drinking 2-3 bottles of vodka a day. He later claimed doctors told him he would have died within weeks if he had continued drinking so heavily. Burton was considered surprisingly fit for a man who smoked 60 cigarettes a day, did no exercise other than swimming, and claimed to have drunk alcohol every day of his life since he was 14.
During filming, Richard Burton was so drunk so often that no one trusted him to remain standing. He is sitting or lying down in most of his scenes. His lines are clearly slurred in some scenes.
Richard Burton and Lee Marvin were both drinking heavily during filming. At one point Burton confided to Marvin that he would like to play a cop in a film set in New York, but couldn't trust himself to do a convincing American accent. (His accent in this film was widely ridiculed, particularly since it wavered throughout the movie). Marvin replied, "But Richard, you're a great actor. You can learn." Burton apparently treasured the compliment for the rest of his life.