Why does she need to make her face "an indifferent mask" and be careful what she says in public?

Katniss, in The Hunger Games, knows
that there will be people that are going to be out to get her, that will hate her, and
will try to sabotage her at all costs.  By making her face "an indifferent mask" in
public, she is trying to insure that she will not offend, anger, or show any emotion in
regard to what is happening.

Facial emotion is a cue as to
how one is feeling on the inside, typically. Katniss does not want anyone knowing how
she really feels. Readers also know how much she hates the Hunger Games and the Capitol.
She does not want that to show either.