One of the fruits of my (and others') efforts to boost the operatic content in Wikipedia is the table of roles and (where available) the première cast in the articles on operas. The article on Roméo et Juliette
http://en.wikipedia.org/wiki/Rom%C3%A9o_et_Juliette gives Marie Caroline Miolan-Carvalho as the Juliette and, hey presto, you can follow the link to her article and discover that she was also the first Marguerite in Faust. In fact, Gounod wrote two other roles for her as well.