I believe he was. I know he worked as an acrobat in Coney Island in New York when he came to the states. His Music Hall ( vaudeville ) experience eventually evolved into acting onstage and then in movies.
Wiki: "Grant's mother taught him song and dance when he was four, and she was keen on his having piano lessons. . . He spent his evenings working backstage in Bristol theaters, and was responsible for the lighting for magician David Devant at the Bristol Empire in 1917 at the age of 13. He began hanging around backstage at the theater at every opportunity." In other words, he would see a lot of performances of such kind growing up.
