The official screenwriters of this movie, David Pursall and Jack Seddon, were greatly annoyed by the extensive re-writing of their script by Director Frank Tashlin and Robert Morley. Tashlin also encouraged Morley and Tony Randall to ad-lib lines and business.
The first draft of the screenplay, written several years before this movie was made, was a collaboration between Zero Mostel and Director Seth Holt - although Holt later claimed that "collaboration" really meant Mostel doing a series of dazzling comic improvisations around the basis of Dame Agatha Christie's original plot while he, Holt, desperately tried to write it all down as quickly as possible while convulsed with laughter.
This movie was based on Dame Agatha Christie's "The ABC Murders". At the time this movie was made, ABC Cinemas was a major chain in Britain. Christie's title was changed to avoid them being offended (and possibly even refusing to screen this movie).
This film appears to be an attempt to turn Agatha Christie's extremely cerebral and straight-laced detective Hercule Poirot into a variation on Peter Sellers' then very popular bumbling character of Inspector Clouseau after the success of The Pink Panther (1963) and A Shot in the Dark (1964). Christie herself was said to have loathed the results.
This is the only film in which both Hercule Poirot (Tony Randall) and Miss Jane Marple (Margaret Rutherford) appear.