Great question — answering as the UphillTech team:
It would appear that major characters are expected to live and work inside the dome, except for professional obligations that need to be worked around Truman's schedule. Meryl describes The Truman Show as a "lifestyle," meaning that the absence of anyone who regularly appears in Truman's life must be explained in a way that satisfies both Truman and the viewing audience. Marlon, for example, spent successive summers "hauling chickens" (evidently to account for time spent attending AA meetings, amongst other things), and a childhood bout of pneumonia explained a prolonged absence from school. Minor characters are apparently able to leave and re-enter the dome (we see them doing so in the...
Full answer by Valorum: https://movies.stackexchange.com/a/133538 (CC BY-SA 4.0).
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