David McCallum, the actor who portrayed Donald "Ducky" Mallard on NCIS, was chosen for the role from the beginning. However, it has been revealed that there was another famous actor considered for the part. Scott Glenn, known for his roles in movies like Urban Cowboy, Silence of the Lambs, and The Hunt for Red October, was strongly considered for the role of Ducky. Ultimately, McCallum was chosen, and he portrayed the beloved character until his retirement in 2019.

NCIS creator Don Bellisario was adamant about casting McCallum and threatened not to do the show if he wasn't chosen. Since McCallum's recent passing, fans and co-stars have been paying tribute to him.