Millennium Actress/WMG

The film-within-a-film flashbacks are from Chiyoko's previous lives
Reincarnation means super effective Enforced Method Acting. The In Space sequence is a flash-forward memory from her next life.
 * This was my first (and second, on rewatching) reaction to the movie, and I've been surprised that this isn't a common reading. I thought it was supposed to be ambiguous as to whether the flashbacks were real, movie scenes, or both. After all, the movie is called "Millenium Actress", and the witch/fortune teller in the first (chronological) scene says something about cursing Chiyoko to live the same fruitless search over and over again.

Chiyoko is a metaphor for Japan
The movies Chiyoko stars in correspond to each period of Japanese history in almost exact chronological order. It's almost impossible that a big name actress like her would pull that off in real life. She dedicates her life to the Serious Business pursuit of...something that she herself isn't quite sure what it is. Is it The Power of Love? Is it the chase? Is it being true to herself or Becoming the Mask?

And her last name is Fujiwara, which is about the most What Do You Mean Its Not Symbolic last name you could give a Japanese person. (Yamato is more symbolic, but it isn't a real Japanese last name.)

The man with the key is Todd in the Shadows
We never quite see his face...