@Quasarbearcat I don't think you need to be gesticulating the entire duration but you do need to be capable of providing the components. If you get tied up or something while casting Simulacrum, the cast is interrupted since you can no longer provide the somatic component.
@Quasarbearcat The way long cast times work is that you're effectively spending every action each turn attempting to cast the spell. Outside of combat, that is abstracted away, but you're effectively stuck trying to cast the spell and having to provide the components for the cast duration.