Look at when the musical is taking place. Daytime showings are typically more relaxed and casual, and you can opt to wear jeans and a shirt or go for a casual business look. For nighttime showings,a business casual look will work.
Select a top garment to wear. For men this can be a neutral-colored button up shirt such as black or white or a striped button up shirt. For women this can be a button up blouse with long or short sleeves or a tailored sweater.
Select a bottom that compliments your top piece. For men stick with slacks, preferable in black, brown or khaki. Women can opt for pants in similar colors or skirts that are no shorter than mid-thigh in similar colors. Women can also opt for patterns like floral or stripes. just ensure they aren't too busy or loud.
Pair your outfit with nice dress shoes. Men can opt for leather or suede shoes to match their pants. Women can wear heels or flats that match their pants or have a complementing color to the rest of their outfit. Stick with heels no higher than three inches in keeping with a business casual look.
Select a few accessories to dress up your look. Don't overload but pick one or two. Men can add a complementing tie, cuff links or silver or gold watch. Women have a few more options, such as a necklace, earrings, a watch, dress scarf, bracelets or brooches. Just make sure accessories aren't overly flashy or distracting.