OK, I read the book so I can provide you with some info.
John Kander and Fred Ebb wrote the song "My Own Best Friend" as a solo for Roxie at the end of Act II, however, Fosse decided that it would be a duet and since he was the director he had the last word (People on this board have said that the reason Fosse made it a duet for Gwen Verdon and Chita Rivera was because Verdon was too weak vocally to handle another solo, this is not discussed in the book). When Verdon got sick and Liza Minnelli replaced her as Roxie Hart, Fred Ebb (a long time friend of Minnelli, almost a father figure) asked her to ask Fosse to let her perform the song as a solo, since Minnelli apparently had a way with Fosse and Chita didn't have problems with not performing the song, then the number was re-conceived as a solo for Roxie.
Read the book though! It's an awesome read! No need to take off the shirt either! (sorry if the commentary offends you coming from a man)
"Some people can thrive and bloom living life in a living room, that's perfect for some people of one hundred and five. But I at least gotta try, when I think of all the sights that I gotta see, all the places I gotta play, all the things that I gotta be at"