• Bob Denver as Willy Gilligan, the hapless first mate of the S.S. Minnow.
• Alan Hale Jr. as Captain Jonas Grumby ("The Skipper"), the captain of the S.S. Minnow
• Jim Backus as Thurston Howell III, a Wall Street millionaire
• Natalie Schafer as Eunice "Lovey" Howell, Thurston's wife
• Tina Louise as Ginger Grant, a Hollywood movie star
• Russell Johnson as Professor Roy Hinkley, Ph.D. ("The Professor")
• Dawn Wells as Mary Ann Summers, a wholesome farm girl from Winfield, Kansas, who won the trip and tour in a lottery
• Charles Maxwell (uncredited) as the voice of the recurring radio announcer

The pilot episode, "Marooned," was filmed in November 1963. The pilot featured seven characters (as in the series), but only four of the characters—and their associated actors—were carried forward into the series: Gilligan (Denver), the Skipper (Hale), and the Howells (Backus and Schafer).

Because of the three significant character and casting changes between the pilot episode and the first series episode, the pilot was not shown before the series first aired on September 26, 1964. The original pilot eventually aired over 29 years later on TBS.

The three characters who did not carry forward from the pilot were two secretaries and a high school teacher. In the pilot, the scientifically inclined Professor was instead a high school teacher played by John Gabriel. Ginger, the movie star, was still red-haired, but she worked as a secretary and was played by Kit Smythe. She was more sarcastic than the later incarnation. Mary Ann, the Kansas farm girl, was instead Bunny, Ginger's co-worker, played as a cheerful "dumb blonde" by Nancy McCarthy.

The pilot's opening and ending songs were two similar calypso-styled tracks written by John Williams and performed by Sherwood Schwartz impersonating singer Sir Lancelot. The lyrics of both differ from those of the TV series, and the pilot's opening theme song is longer. The short scenes during this initial music include Gilligan taking the Howells' luggage to the boat before cast-off and Gilligan trying to give a cup of coffee to the Skipper during the storm that would ultimately maroon the vessel.

After the opening theme song and credits end, the pilot proper begins with the seven castaways waking up on the beached SS Minnow. It continues with them performing various tasks, including exploring the island, trying to fix the transmitter, building huts, and finding food. Contrary to some descriptions, the pilot contained no detailed accounts of the characters' backgrounds. It concludes with the ending theme song and credits. The background music and even the laugh tracks of the pilot appear nearly identical to those used during the series.
The first episode broadcast, "Two on a Raft," is sometimes incorrectly referred to as the series pilot. This episode begins with the theme song performed by the Wellingtons and then the same scene of Gilligan and the Skipper awakening on the boat as in the pilot (though slightly differently cut to eliminate most shots of the departed actors) and continues with the characters sitting on the beach listening to a radio news report about their disappearance. No equivalent scene or background information is in the pilot, except for the description of the passengers in the original theme song. Rather than reshooting the rest of the pilot story for broadcast, the show proceeded. The plot thus skips over the topics of the pilot; the bulk of the episode tells of Gilligan and the Skipper setting off on a raft to try to bring help but unknowingly landing back on the other side of the same island.

The scene with the radio report is one of two scenes that reveal the names of the Skipper (Jonas Grumby) and the Professor (Roy Hinkley); the names are used in a similar radio report early in the series. The name Jonas Grumby ap…

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The shipwrecked castaways are desperate to leave the island, and various opportunities frequently present themselves but invariably fail, nearly always due to some bumbling error committed by Gilligan. Sometimes, this results in Gilligan saving the others from some unforeseen flaw in their plan. The only episode in which the entire cast leaves the island (albeit temporarily) is "The Friendly Physician" aired in Season 2.

Most episodes of Gilligan's Island use variations of five recurring basic plots:
• Life on the island. A running gag is the castaways' ability to fashion various useful objects from bamboo, coconuts, gourds, vines, and other local materials. Some are everyday items, such as eating and cooking utensils, while others (such as a dental drill and a remarkably efficient lie detector apparatus) are stretches of the imagination. Russell Johnson noted in his autobiography that the production crew enjoyed the challenge of building these props. These bamboo items include framed huts with thatched grass sides and roofs, bamboo closets strong enough to withstand hurricane-force winds and rain, a communal dining table and chairs, Gilligan's hot water pipes, a stethoscope, and a pedal-powered car.
• Visitors to the island. Another challenge to a viewer's suspension of disbelief is the remarkable frequency with which an assortment of people visit the remote, uncharted island, all of whom either refuse or fail to help rescue the castaways.
• Dream sequences in which one of the castaways dreams they are some character related to that week's storyline. All of the castaways appeared as other characters within the dream. In later interviews and memoirs, nearly all the actors stated that the dream episodes were among their favorites.
• A piece of news concerning one or more of the castaways is heard over the radio and causes distress or discord among them.
• The appearance or arrival of unusual objects to the island, such as a World War II naval mine, an old silent motion picture camera and costumes, a crate of radioactive vegetable seeds, plastic explosives, a robot, a live lion, a jet pack, or a wayward "Mars Lander" that the scientists who launched it think is sending them pictures of life on Mars.
Most of the slapstick comedic sequences between Hale and Denver were inspired by Laurel and Hardy, particularly when Hale breaks the fourth wall by looking directly into the camera, expressing his frustration with Denver's clumsiness, as Oliver Hardy often did.
Although most episodes do not have guest stars given the characters are on an uncharted, unknown island, a remarkable number of people manage to find their way to the episode - and to leave without helping the castaways also escape. Among the actors who guest starred were Larry Storch, Kurt Russell, Rory Calhoun, Zsa Zsa Gabor, Hans Conreid, Don Rickles, John McGiver, Denny Miller, Phil Silvers, Richard Kiel, and pop group The Wellingtons (folk group).
One of the trademarks of Gilligan's Island was its frequent use of dream sequences. The showrunners used this device to keep the setting from becoming redundant and showcase the cast's acting talents. Many episodes that feature dream sequences are ranked among the show's most memorable episodes, as most of them parodied or drew inspiration from works of literature, film, and other television shows of the day.

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