The two booksellers Corso encounters in Toledo are actually the same actor, José López Rodero.
Writer/director/producer Roman Polanski used a motion capture rig to use the same actor twice.
The same man appears again later, playing two workmen cleaning out the bookstore.
Rodero was an assistant director and production manager, not a professional actor. He was hesitant to accept these multiple parts.

Director Cameo: The taxi cab driver that drops Corso off at the airport is played by Roman Polanski.

The New York sequences were shot in Paris as Roman Polanski could not set foot into America because he was still wanted for his 1977 sex crime charges.
The exterior locations were redressed with American-related details and the skyscraper seen in the opening
and in Balkan's office were Translite material taken from Manhattan.

The voice of one of the Ceniza twins (Pedro Ceniza) is dubbed by Roman Polanski himself.

The little girl that stares at Corso at the Paris Airport is played by Roman Polanski's daughter Morgan.