As loudly as possible.
A bit early.
Just as the performance is beginning.
Only in time to catch your friend's performance.
Discuss the performance.
Sit quietly and listen.
Answer your phone quickly and speak quietly.
Leave the room to use the phone.
Turn the phone off, or better yet, leave it in the car.
The performance is completed and the conductor faces the audience.
Your friend has completed his part.
Any time something is done well.
You should leave your seat and rush up to the stage to find your friend.
Exclaim loudly how tired you are.
Sit and wait until everyone has cleared the stage.
Critique the performance.
Offer your praise.
Offer your sympathy.
Sing, hum, or whistle along.
Sit quietly and enjoy the music.
Yawn loudly because it's boring.