Richard "Dickie" Attenborough greets the detectives as "Darling." This was a real-life quirk of Attenborough's which he admitted he would often use if he forgot someone's name.
Most of the police force are busy dealing with the "Rillington Place" murders. Sir Richard Attenborough (the lead actor in the play "The Mousetrap") played the serial killer Reginald Christie in 10 Rillington Place (1971), based on those real-life crimes. In addition, when constable Stalker is calling women from the victim's little black book, she calls a Beryl and a Geraldine. These are the names of the mother and daughter Christie killed and blamed on Beryl's husband.
The announcer heard on the radio when part of The Mousetrap is being performed in this film is the same recording used in the actual stage play. That recording was made by the late actor Deryck Guyler in 1952 and is still in use to this day. Also the viewer can briefly see the clock above the fireplace. This is the only surviving prop from the original opening night of the show on October 6th 1952 and is still in use to this day.
While questioning the concierge at the Savoy, Stalker asks him what part of France he is from, to which he replies "Belgium." This is a reference to Dame Agatha Christie's famous character Hercule Poirot, who is Belgian, but is often mistaken for being French.
"The Mousetrap" is a real murder mystery play by Dame Agatha Christie that premiered at the Theatre Royal in Nottingham on October 6, 1952 and has continued to run to this day, making it the longest-running play in the world. Sir Richard Attenborough and his wife Sheila Sim were the first actors to play Sgt. Trotter and Mollie Ralston, respectively. A film adaptation was announced in 1959 to be produced by Edward Small and Victor Saville for United Artists and starring Tyrone Power and Maria Schell, but the film was never made. (Small produced the previous Christie adaptation Witness for the Prosecution (1957) starring Power.) There eventually were two film versions of the play - the Bengali Chupi Chupi Ashey (1960) and the Russian Myshelovka (1990). As portrayed in the film, The Mousetrap cannot be made into an English language film until at least six months after it has closed on the West End. The longest break between shows was 14 months during the COVID-19 pandemic.