“Oh, this is just beyond,” Ms. Walsh said.

The corseted white gown, made of silk gazar and organza, was hand-embroidered with a black rose pattern and covered in sequins that make the dress look liquid when it moves. The full skirt is supported by layers of tulle and other fabrics. The dress took 190 hours to create, with 80 hours devoted to the embroidery alone.

Sophia Loren was the inspiration for the look, Ms. Walsh said, pointing out the square halter neck of the dress. “We wanted old Hollywood glamour,” she said. “Something incredibly sensual, powerful, feminine and whimsical to create this movie star kind of moment, and the color and fabrics were influenced by that.”

Ms. Feldstein’s voice filled the room when she spoke, clear and excited. “It’s not in charge of me, I’m in charge of it,” she said, laughing as she walked in a circle around the room after being pinned and prodded for fit adjustments.

“It makes her look like a beam of light,” Ms. Walsh said.

Ms. Feldstein hopped from side to side, angling her face as she tested the weight of the dress. “It’s heavy, but my dress in ‘Hello, Dolly!’ was 20 pounds and we did a whole polka in it,” she said, referring to her Broadway debut in 2017. “Trust me, I’ll be O.K.”

Soon came the ultimate test: Could she sit in it? Ms. Feldstein perched on a chair, her skirts billowing around her.

“Good luck to my mom sitting next to me,” she said of her mother, who is her date at the ceremony. “She’ll be using it as a blanket.”