The main cast regularly attend script meetings in the OmniCam Ecosphere which means that they can certainly leave Seahaven, albeit remaining inside the dome itself.
In the Truman Show Behind the Scenes mockumentary, Truman's "mother" (Alanis MontClair) claims to be effectively a prisoner of her punishing schedule. She needs to be available when Truman calls, which means she needs to stay in her house almost all of the time.
"I'm a prisoner, and yet I have the financial means to do anything. I mean, I could buy the Ritz Hotel in Paris. Of course, I just couldn't go there"
It would seem likely that the main cast characters are required to stay inside the dome 24/7, excepting holidays. Extras only need to be available during their shifts, which would suggest that they leave while Truman is asleep and return (to "first positions") the following day.