Only the River Flows is based on Yu Hua's story Mistakes by the River. Director Yimou Zhang initially wanted to make a suspense film based on this story, back in the 1990s. Per Yu Hua, they wrote for many days but didn't know how to film it. Yu Hua gave Yimou Zhang another story, To Live, instead, which was made into same named movie Huo zhe (1994) , and subsequently won the Grand Prize of the Jury of Cannes Film Festival in 1994.
Yilong Zhu decided he should put on weight to play his character Ma Zhe, based on his study and observation of local retired policemen. The police investigators had to stay up late and often ate at late hours, which would be a major factor for weight gain from overwork. The first thing he did after arriving at Nanfeng, the filming site (which was also the day after he won his Golden Rooster Awards), was to put on weight. He did eat a lot and complemented on Nanfeng's local cuisines.
Per actor Yilong Zhu in an interview with ELLE, The most difficult part was to find a way to establish his character, Ma Zhe, how the character should look like. He had no answer until he saw a picture of Yu Hua, the author of the original story. Yu Hua was in a picture with mustache, on a snow day, with long hair. That inspired Yilong Zhu the look of Ma Zhe.
After his observation and studies, Yilong Zhu suggested to director, and they decided the character Ma Zhe should be heavy at the beginning and lose weight through the story. Yilong Zhu ate five meals with extra carbohydrates. After a month he put on at least 20 lbs and reached 180 lbs, which was his all time high. During the 2-3 months' filming, he lost 22 lbs as what he designed for character over the story.