Acting for Two: Ryosuke Miura, of course, plays both Ankh and Shingo Izumi. Both characters are only active for a very brief period in #42 after Ankh absorbs Lost Ankh and regains his full Greeed body.
Marie Kai plays both Chiyoko and Dr. Maki's sister Hitomi in flashbacks.
Like Ankh, The Shogun and the 21 Core Medals' Big Bad Gara possesses an ordinary woman, meaning we get her actres Miki Sakai playing both roles.
Actor Allusion: In the Shogun and Core Medals movie, we see a guy carrying a guitar who takes up the Perky Female Minion's offer and get a samurai hairdo. The actor in question? Mitsuru Karahashi, who played the guitar as Naoya Kaidoand played a (fallen) samurai, Juuzo.
Dawson Casting: Or something like it. In Let's Go Kamen Riders,Isao Sasaki plays the "Man in White," who is the adult version of Naoki, separated from the Riders in the 1970s and eventually too important to history to go back and retrieve. However, since he was born in 1942, Sasaki was in his thirties in the year 10-ish-looking Naoki was stranded in.
Dyeing for Your Art: Ryosuke Miura (who plays Ankh) had his hair dyed and styled to play the role. His more "normal" hairstyle as Shingo Izumi is a wig.
Fan Nickname: Japan has gone back to the tried and true method of numerical nicknames, giving Shintaro Goto the name "5103" (5=Go, 10=Tou, 3=San).
Some English-speaking fans call Goto "Cakebitch". Also "Broto", in reference to a fansub group using this as a "translation" for the times when Date addresses him as "Goto-chan".
And in #27 Megumi is one of a pair of female bikers who fight the Yummy... and win.
In the 21 Core Medals movie, Yoshimune is played by Ken Matsudaira, who played the very same character in the 25-year Jidai Geki TV series Abarenbō Shōgun.
Isao Sasaki makes an appearance in the Let's Go Kamen Rider film as the adult version of one of the children Eiji and Kotaro Nogami befriend, who, oddly enough, is also the father of another one of those children.
The Dinosaur Yummy costumes from Movie War Core are re-used as Dr. Maki's first Yummies in #32.
Throw It In: Eiji and Ankh's imfamous playing with toy animals at the end of one of the net movies.
Word of God: The Kougami Foundation is based on Smart Brain as Smart Brain was the most realistic company seen in Kamen Rider with Satonaka being a toned-down version of Smart Lady. Now show of hands who wants to see Kougami be used to make the Rose Yummy or Horse Yummy?