Prior to making the film, star Burt Lancaster, despite being quite athletic, had a fear of water and took lessons from Olympian and UCLA water polo coach, Bob Horn. Lancaster trained at the pool at his own house and one at UCLA.
Star Burt Lancaster worked out continuously throughout the production of the film to maintain a high level of fitness. Lancaster did numerous push-ups and sit-ups every morning before each shoot to stay lean and solid. He gained twenty pounds of muscle by doing weights, running, karate, and aerobics.
Although the critical response at the time was mixed, critics were unanimous in their praise of Burt Lancaster's physique, especially considering he was 52 at the time of filming.
John Cheever: Uncredited, the author of the source short story "The Swimmer" as a man at the pool party.