With the help and support of director Robert Lewis (who had once danced with her) she suggested changes in the script for Brigadoon, at first to strip it of some of its sentimentality, but basically to add more action through dance. They added the wedding dance and the sword dance at the end of the first act. The chase scene that begins the second act was also their invention, and wasn't in the original script. De Mille developed the funeral sequence from a line or two. The new sequences required additional music, much of it arranged by a specialist in music for dance, Trude Rittman.
That Brigadoon became a hit is all the more impressive considering what other shows were on Broadway at the same time. They included Annie Get Your Gun, Finian's Rainbow and the still-running de Mille choreographed shows, Oklahoma! and Carousel. De Mille continued to choreograph for Broadway, including for another Lerner and Loewe show (Paint Your Wagon) and several revivals of Brigadoon and her other hit shows.
[Some of this information is from 1992 liner notes by Miles Kreuger to Brigadoon on the Angel label.]