Martin Sheen also played the President in the mini series "Kennedy" and in another character's vision in 1983's The Dead Zone
Martin Sheen's daughter, Renee, plays President Bartlet's assistant Nancy. Since she doesn't use her father's stage name, Nancy is credited by her birth name "Estevez."
Janel Moloney (Donna Moss) was originally intended to be a rarely seen or heard background player (similar to Tobey and Sam's assistants). But very quickly, her banter and rapport with Josh became a fan favorite and she was promptly made a regular cast member and given much more screen time. When Mandy was dropped from the show after season one, Moira Kelly's name and image were dropped from the opening credits, replaced by Janel. As they say, "There are no small parts. Only small actors."