Many of the polar cards say things like " Play during your opponent's turn, after characters have been committed to stories. "

If an opponent chooses not to commit any characters to stories, the rules say

If the active player decides not to commit at least one character to a story, the phase ends and the player's turn is over.

So, if the active player commits no characters, does that mean there is no opportunity for an opponent's polar card to activate?

If a player not commits characters, her turn ends. And you have not opportunity to play those "Action".