Unless the script calls for it AND you’re the actor…
If you look anything but happy on set, you had better be an actor because no one is paying you to be dramatic.
This advice was borrowed directly from an article by Barbara Freedman (author of the wonderful book that you should read: Make Your Movie).
As you work your way up the ladder, you’ll have to put up with a lot. Smile! Things only get better. Barbara tells a great story in her article on IndieWire (link) about a PA who had to do a lunch run. Understandably, the PA was less than thrilled with this task. This was noticed by the producer who quickly put a stop to that kind of attitude. Read Barbara’s book for more!
You are certainly allowed to be unhappy – the work is hard, the hours are long, and you’re away from your family. But do your job and be happy about it. Worry about what you are able to control and learn from the mistakes of others.
Keep your interactions positive. The people in charge will break this rule, but it is never appropriate. Politics is the game and it’s up to you to learn how to play it.