Gregory Peck was offered the lead role of McGarrett. He turned it down.
Kam Fong, who played Det. Chin Ho Kelly (1968-78), was an actual officer with the Honolulu Police Department who served from 1946 to his retirement in 1962 to take up a career in real estate.
Other than Wo Fat, other notable adversaries for McGarrett that appeared in more than one episode included mob bosses Henore Vaschon (played by Harold Gould) and Tony Alika (played by Ross Martin), pimp Big Chicken (played by Gavin MacLeod) and the Robin Hood-like Lewis Avery Filer (played by Hume Cronyn).
Jack Lord was the only one of the cast to stay with the series during it's entire 12 year run. Kam Fong (Chin Ho Kelly) left after the 10th season. James MacArthur (Danny Williams) left after the 11th season.
The character of Duke Lukela first appeared as an HPD sergeant before becoming a Hawaii Five-O investigator.
The syndicate that Tony Alika headed was called "Kumu".
McGarrett finally caught Wo Fat in the final episode of the series. However, at the end of the episode, Wo Fat can be seen digging into his boot and taking out a file leaving it open for a possible reunion episode.
'Magnum P.I.(1980)_' began production soon after this series wrapped its last episode. In order to keep some sort of continuity, reference to characters from this show were included in early episodes of Magnum. However, a plan to have Jack Lord appear as McGarrett never came to pass. Lord retired from acting after the series went off the air. Second half of first episode of 'P.I. (1980) Magnum' makes reference to McGarrett when speaking about the bad assignments for Dan: "Putting Dan on this stuff is like making McGarrett the meter maid on Waikiki."
McGarrett was a Navy Intelligence officer before he became head of Five-O. In fact, he was in the reserves and went on active duty from time to time to assist the Navy or Coast Guard on special cases.
In the episode "The Singapore File" McGarrett flies to Singapore to retrieve a witness, and returns to Honolulu. Singapore was actually downtown Honolulu. At the end of the episode, they are at a temple in Manila; they were actually at the Valley of the Temples in Kaneohe, Hawaii.
McGarrett actually caught Wo Fat in one encounter, but was forced to give him up because the Chinese government wanted him back in trade for a downed U2 pilot.
Zulu (Det. Kono) was the first of the regular cast to depart the show. He left in the 4th season.
McGarrett has a sister that lives in California.
Before joining Five-O, Danny attended the University of Hawaii and then transferred to the University of California where he graduated with a degree in Criminology.
McGarrett often referred to Kono as "Big Kanaka".
McGarrett often referred to his secretary, May as "Love".
McGarrett was a Korean War veteran.
McGarrett supposedly had his office in Iolani Palace; the actual palace used by the last kings and queens of Hawaii. As of 1969, the Palace had fallen into disrepair after years of abuse and neglect. Restoration was begun, and the Palace can now be toured for a $20 fee.
Besides government intelligence chief Jonathan Kaye, other recurring characters include policewoman Sandy Welles, Doctor Grant Ormsbee, and hang glider pilot Molly Taggart.
Danny was played by 'Tim OKelly in the pilot.
Several actors played different roles in various episodes before becoming recurring cast members.
Al Harrington and Herman Wedemeyer both appeared in different roles on the show before assuming the roles of Ben and Duke respectively. Wedemeyer was in the very first episode playing Honolulu police Lt. Balta.
Despite the attention that Hawaii Five-0 brought to Hawaiian state law enforcement, Hawaii is the only state that has no state police agency.
The hula dancer in the opening montage is played by Helen Kuoha-Torco, now a professor at Windward Community college.
Opening montage consists of: Breaking wave, which first appeared in the film Surfari (1967); a hula dancer, played by now-college professor Helen Kuoha-Torco; the "Lady Columbia" statue at the National Cemetary of the Pacific. Model Elizabeth Logue running down the beach; close-up of Ms. Logue from a side view; then 13-year-old local resident Mel Kinney; Elizabeth Logue turning toward camera; red neon sign, formerly the Tops restaurant in Waikiki Diamond Head; flashing blue light; Aloha Tower, Honolulu; Ilikai Hotel.
For the ninth season, the Five-O offices moved out of the Iolani Palace and into an office building down the street (they moved back for the tenth season). Part of the reason was that the Iolani Palace itself was undergoing a facelift; the other part was that CBS was building a new studio for the series, which wasn't ready in time for the early episodes. The Five-O office scenes were actually filmed in the office building, which was introduced in each ninth-season episode by showing the Iolani Palace with scaffolding over its facing and then quickly panning down the street to the office building. Sometimes an exterior shot of the office building would show McGarrett pacing in front of his "new" office window.
The Five-O in the series' title pertains to Hawaii's rank as being the 50th state to enter the union (August 21, 1959).
In a few early episodes, Richard Denning (who played Governor Paul Jameson), also played Attorney General Phil Gray.
Jack Lord (Detective Steve McGarrett) is the only actor to appear in all 283 episodes of the series.