There are certain etiquette rules that are observed when visiting a theatre.    

When visiting the theatre it is better to wear the classic suit of neutral colour in order not to attract attention of the audience who comes to enjoy the performance but not your eccentric clothes. Women can decorate their dresses or suits of austere style with modest jewelry. Men should wear dark suits. It is important to have the change of shoes when the weather is bad.  
According to etiquette rules men sitting in the boxes and the orchestra stalls should wear dress suits and women should do evening dresses. For the opening night festive attire is acceptable and for everyday performance informal clothes are.  

Being late for a performance is inadmissible. Arrive early, or on-time. If you arrive late you shouldn’t distract the audience and go to your seats treading on their feet. Wait until the act of the performance is finished and take your seats in the intermission. You should go along a row with the face directed to other spectators and excuse for the trouble.
As at the restaurant a man should accompany a woman pointing the way to the seats. In the cloakroom a man should take off his hat and over clothes and then help a woman to take off. According to etiquette rules a woman is allowed to wear a hat in the restaurant or cafe but at the theatre it should be taken off because the brim of the hat may obstruct the view of the stage to those behind it. After taking the hat and over clothes off a woman can come to the mirror to smooth her hair and look if her appearance is all right. It is inadmissible to make up, to put on lipstick and to straighten out the hem of the dress in the cloakroom. It should be done in the ladies’ room. While a woman looks at herself in the mirror a man should wait for her patiently aside. The only thing he can allow himself is to buy a theatre program and to read it.
If seats are in the circle a man should go half a step ahead of a woman while ascending and half a step behind while descending. At the stalls a woman takes a seat after a man. If four persons (two women and two men) want to visit a performance at first one of the men takes a seat then women sit down and then the second man does. It isn’t necessary for wives to sit near their husbands. A gentleman always gives the preferred seat to a lady for best viewing.
To sing along with the cast, to tap with hands and to stamp with feet in time of music is considered to be ignorance.
You shouldn’t talk to neighbors, eat and rustle with wrappers during a performance. If you have cough or running nose don’t cough and blow your nose in the hall. You should apologize to your neighbors and leave the hall. The same should be done if your child starts to be noisy.
During the intermission one can have a snack in the bar but it should be taken into account that intermission lasts approximately 15-20 minutes.
It is rude to leave during the performance or several minutes before the end of performance. A polite person and a thankful spectator will wait for a moment when he is allowed to thank actors and musicians for the performance with wild applause.

Clapping rules

Appropriate times to clap are the following:

• at the theatre: after the last act, after especially well-played aria or scene, during the appearance of a popular or outstanding actor on stage;
• at the concert: during the entrance of a conductor and soloists, after song performance by a soloist

You shouldn’t clap:

• during theatrical performance;
• during a pause between parts of musical, chamber or symphonic piece.