Period pieces, by and large, are meatier roles. The quiet nature of the films allow for characters to dwell in silent thought and reveal more about their character than a page of dialogue. Keira's good at this. Very few actresses are.
Besides, she's only 23. She's got a lot of time to do a lot of different roles.
"Purgatory's kind of like the in-betweeny one. You weren't really shit, but you weren't all that great either. Like Tottenham."
I'll try being nicer...if you'll try being smarter.