Paul Henreid could not play the cello. While he was able to fake it in the long shots, to achieve the illusion in close up, he wore a special jacket with no sleeves and holes for two real cellists to insert their arms - one to bow, and one to accurately finger the music - while seated behind him, out of shot.
Paul Henreid's cello-playing was dubbed by Eleanor Aller (Mrs Felix Slatkin) while she was pregnant with Frederic Zlotkin. Her father, Gregory Aller, coached Henreid in plausible bow movements.