If he says try that scene again, but make your character more cheerful, try your best to do that. At the very least, even if you don't have a good instant sense of what the director wants, or aren't quite sure how to suddenly make your character seem more cheerful, above all do something different. Don't just read it the same way you had been reading the lines. The director wants to see if you can take direction, so above all, make the subsequent reading somehow different. 9.Some directors ask for improvisation.
Some actors are nervous and uncomfortable with improv.
Above all, maintain good stage sense, and for God's sake say something, even if it doesn't particularly fit the character. Move across the stage if you get the chance, and when you do move, move properly and confidently. Don't be afraid to move. Actors frequently line up in a straight line across the front of the stage, stick their noses in their scripts and read the lines. If you have the chance, cross the stage at one point, then turn and look back at the other actors.