Gilligan, of the show's title, was the hapless first mate of the S.S. Minnow. Other characters are the Skipper (referred to as Jonas Grumby in the first broadcast episode), the Professor (referred to twice as Roy Hinkley), the millionaire Thurston J. Howell III and his wife Eunice, nicknamed Lovey, movie star Ginger Grant, and Kansas farm girl Mary Ann Summers.
Bob Denver was not the first choice to play Gilligan. Actor Jerry Van Dyke was offered the role, but he turned it down, believing that the show would never make it. He choose instead to play the lead in "My Mother, the Car". The producers then looked to the lovable beatnik, Maynard G. Krebbs from the The Many Loves of Dobie Gillis. Denver embraced the role.
Alan Hale, Jr. both embodied and relished the role of the Skipper. A long time actor in B-westerns, Hale so loved his role as the Skipper, that long after the show was off the air, he would still appear in character in his Hollywood restaurant.
Jim Backus was well known by the time he took the role of the Millionaire, Thurston Howell, III. He was perhaps best known as the voice of the cartoon character Mr. Magoo. He used some of the voice inflections and mannerisms of Magoo in the role of the Millionaire. He was well known for his ad-libs on the set.
Natalie Schafer had it written into her contract that there were to be NO close-ups of her during filming. This was perhaps due to her advanced age. She was 62 when the pilot was shot. Reportedly, no one on the set or in the cast knew her real age, and she refused to divulge that information. Originally she only accepted the role because the pilot was filmed on location in Hawaii, and she looked at the job as nothing more than a free vacation as she was convinced that a show this silly would never get on the air.
Tina Louise, as the character of Ginger Grant, created a version of the quintessential Hollywood star and even her name, an amalgam of Ginger Rogers and Cary Grant, was a homage to Hollywood's past. When regular shooting began, Tina Louise clashed with producers, because she believed she was the main focus of the show, despite it being titled "Gilligan's Island". In addition, her character was originally written as a sarcastic and sharp-tongued temptress but Louise argued that this type of character was too extreme and refused to play it as written. A compromise was reached and Louise agreed to play her as a cross between Marilyn Monroe and Lucille Ball. The evening gowns and hair style used was designed to recreate the look of Myrna Loy. Louise continued to clash with producers and was the only cast member who refused to return for any of the TV movies that followed the series cancellation, saying that the role had destroyed her career as a serious actress. However after the series ended she did appear in a reunion of the series cast on a late night TV talk show.
Dawn Wells was a former Miss Nevada when she auditioned for the role of Mary Ann. Her competition included Raquel Welch. She was part of the replacement cast that was hired after the pilot was shot and the network wanted the roles of the professor and the 2 girls recast.
Russell Johnson took over the role of the Professor from John Gabriel. The network thought Gabriel looked too young to have all the degrees attributed to the professor. Ironically the Professor was in reality just a high-school science teacher, not a University professor.
Charles Maxwell was the uncredited voice of the "Radio Announcer", whose plot-advancing radio bulletins were eagerly tuned in to by the castaways in many episodes.