The idea that Bugsy Seagel dreamed up the Las Vegas Strip on his own is only partly true. The Flamingo was the third major property on the Las Vegas Strip (behind the El Rancho and Last Frontier), and it wasn't even originally Bugsy's idea. The idea for the Flamingo came from degenerate gambler Billy Wilkerson, who intended to model it after his Los Angeles nightclubs. Bugsy Seagel became Wilkerson's partner and eventually took over the project. He later injected a lot of his own ideas into the project, but the original idea was not his.
After working on this film, Warren Beatty and Annette Bening were married. They'd met several years before when she auditioned for another Beatty production. They met again when she was seriously considered for the role of Tess Trueheart in Dick Tracy (1990).
While the film suggests that Bugsy Siegel and Virginia Hill first meet on the movie set, they had actually met several years earlier in real life. At the time, Hill was dating Joe Adonis (portrayed as Joey A. by Lewis Van Bergen in this film) when she and Bugsy had an affair, thus explaining the animosity Bugsy and Joey have for one another throughout the film.
In the late 1970s Jean-Luc Godard became obsessed with the story of Siegel and planned to make a movie about him. He wrote a screenplay called, simply, "The Story", and planned to cast Robert De Niro and Diane Keaton in the Siegel and Hill roles. He dropped this plan when Keaton lost interest and then turned his attention to Every Man for Himself (1980) as his return to commercial filmmaking. An excerpt from his draft of the script can be found in the 1985 edition of the book "Godard on Godard."
The film shows Siegel watching a screen test of himself. In real life, Bugsy Siegel made many friends amongst the Hollywood elite, asked for and had a screen test. The footage no longer exists, like so many other screen tests, yet the legend of Siegel's attempt to break into showbiz lives on.