Catalyzes the reduction of the delta-24 double bond of sterol intermediates during cholesterol biosynthesis. In addition to its cholesterol-synthesizing activity, can protect cells from oxidative stress by reducing caspase 3 activity during apoptosis induced by oxidative stress. Also protects against amyloid-beta peptide-induced apoptosis. cholesterol + NADP(+) = desmosterol + H(+) + NADPH H(+) + lanosterol + NADPH = 24,25-dihydrolanosterol + NADP(+) 5alpha-cholest-8-en-3beta-ol + NADP(+) = H(+) + NADPH + zymosterol Steroid biosynthesis; cholesterol biosynthesis. Interacts with DHCR7; this interaction regulates DHCR7 activity. Belongs to the FAD-binding oxidoreductase/transferase type 4 family.