The name change tells you something. Ramon Estevez got doors slammed in his face, so he became Martin Sheen. He broke through in Badlands (1973) as a charming amoral killer, then Coppola cast him in Apocalypse Now after replacing Harvey Keitel mid-shoot. During production in the Philippines, he had a heart attack at 36, told Coppola it was heat stroke to keep the film going, and had his brother film stand-in shots from behind while he recovered. The part that nearly killed him is still the role he's most proud of.
Aaron Sorkin wrote President Bartlet as a supporting character. Sheen turned four-to-five planned episodes per season into seven seasons, a Golden Globe, and two SAG Awards. The role landed in 1999 when audiences wanted to believe smart, decent people ran the country. Now in his mid-80s, he still works steadily: Grace and Frankie ran through 2022, and he's in production on a The Way sequel with son Emilio Estevez. The image has settled: principled, Catholic, consistently present.
He told an Oxford audience in 2009 that he'd been arrested roughly 66 times for civil disobedience. He started young: at 14 he organized a caddies' strike at an Ohio country club. The faith driving all of it is serious Catholicism, not just branding. He's said activism is what keeps him alive, not acting. Sea Shepherd named a research vessel the RV Martin Sheen in his honor. In 1989, Malibu appointed him honorary mayor. He declared it a nuclear-free zone and a sanctuary for undocumented immigrants and the homeless.