Evening Primrose (1966)

A young poet gets the brilliant idea to live in a department store, hiding by day, and courting his muse by night where it's quiet, and he can have all his needs met. But, to his surprise, he learns his brilliant idea's not exactly original; there are other residents who dodge the night watchmen, and who keep their existence secret at all costs. And one of them is a young woman who wants to leave, but is too frightened to go. And Charles finds that he wants to show her the larger world outside.