Why did Tanvi's mind go "completely empty"?

Tanvi had only four lines in the school play. She was the messenger who ran onto the stage in the second act, shouted that the king's army was coming, and ran off again. She had practised the lines so often that her little brother could say them in his sleep.

But when the curtain opened on the night of the play, and she saw the hall full of parents and teachers and the bright lights shining in her face, her mind went completely empty. She could not remember the first word. She could not remember anything.

From the side of the stage, she watched the king and queen talking. Her turn was coming closer. Her hands were wet. She thought about running out of the back door of the hall and going home.

Then she noticed Mr D'Souza, the drama teacher, sitting on a stool in the dark corner beside the curtain with the script on his knees. He looked up, saw her face and smiled. He did not say anything. He just pointed at his own chest and then at the stage, as if to say: you know this. It is in here.

Her cue came. Tanvi ran onto the stage. She opened her mouth, and the four lines came out, loud and clear, as if they had been waiting for her all along.

Afterwards, her brother told her she had shouted so loudly that a baby in the third row had cried. Tanvi decided to take that as praise.