David Wayne, Robert Preston, Gig Young, Rex Harrison, Tony Randall, and Dick Van Dyke, were all sought for the role of Henry Orient.

Among the actresses sought for the roles of "Val" and/or "Gil" were Hayley Mills (whose refusal took over a year), Patty Duke, Sue Lyon, 'Laurel Goodwin', Portland Mason, and dozens of unknowns.

Among the numerous hassles that producer Jerome Hellman faced during production were problems with the craft unions and scheduling problems over Peter Sellers's other commitments.

During the making of the film, Peter Sellers actually had problems of his own with a fan stalking him.

In 1967, a musical version "Henry, Sweet Henry" opened on Broadway. Both the film and the musical adaptation were directed by George Roy Hill.

Shot from June to October 1963.

The film premiered at Radio City Music Hall.

The first time Elmer Bernstein scored a comedy.

The official US entry at the 1964 Cannes Film Festival.

The phone Peter Sellers uses in his bedroom is called a Ericofon, made by L. M. Ericsson of Sweden. This is one of the very few foreign phones allowed in the US at the time of filming by the then telephone company, Bell Telephone, which held a monopoly on both telephone service and telephone equipment in the US. Bell Telephone felt so threatened by the unique European design (and possible mass intrusion into "their" telephone network) that they designed the "Trimline" phone as a countermeasure.

The pianist's unusual surname - Orient - was inspired by real-life concert pianist Oscar Levant. The word "levant" means orient in French.

Jack Benny:  Has a wordless cameo as a percussionist during Henry Orient's concert. He gives Henry the trademark Benny "look".