Charlton Heston put a steel rod in his nose to make it resemble Michelangelo's broken one, but Rex Harrison steadfastly refused to grow a beard, even though the real-life Julius II had one during a short period of his papacy.
In the film, the quarries that Michelangelo (Charlton Heston) sourced for the marble for his sculptures were the actual quarries attended by the real Michelangelo at Carrara, Massa-Carrara, Tuscany, Italy.
Heston was asked in an interview, which of all the characters he'd portrayed would he like to have as a dinner guest. He said Michaelangelo, but acknowledged that, as the painter did in the film, Michaelangelo would certainly not show up.
Irving Stone, the author of the book which this movie was based on, lived in Rome for about four years, traveling throughout Italy doing research, including periods working in quarries, even apprenticing to a sculptor.
The first conversation Michaelangelo (Charlton Heston) has with Bramante (Harry Andrews) is to discuss a large block of marble for the carving on Moses, a sly reference to one of Heston's most famous roles, that of Moses, in The Ten Commandments (1956).