Characters
You're supposed to be the leading lady in your own life, for God's sake!
— Iris · The Holiday
Behind the line May contain spoilers.
Our reading
An editorial interpretation.
The speaker insists, with exasperation, that a person ought to be the star of their own life rather than a bystander in it. Leading lady is the role one is meant to claim, and the outburst 'for God's sake' shows frustration at seeing it neglected.
Context reviewed