Rehearsal for Broadway show 'Scandals' in NY City

The video tutorial guides a group of dancers through a practice session. It begins with safety instructions, followed by detailed guidance on maintaining a checkerboard formation while dancing to the song 'Life is Just a Bowl of Cherries.' The instructor emphasizes keeping lines straight and smiling. The session concludes with a final practice and words of encouragement to the dancers.

What should the dancers keep very straight?

What is the instructor's request to the dancers regarding their expressions?