Geek audiences might know Rose Bryne for having played Moira MacTaggert in X-Men: First Class, but comedy fans likely know her for Bridesmaids (directed by Paul Feig), Get Him To The Greek, This Is Where I Leave You, and Bad Neighbours. Horror fans might recognize her from Insidious and 28 Weeks Later. In other words, the Australian actress has been able to handle both comedy, horror, and superhero fare before (and while the Ghostbusters aren't "super" heroes, they are heroes). She's also working with Feig in the upcoming action-comedy Spy, so she still has a strong working relationship with him. She could very likely pull of the nerdy, science-y type - the female version of Egon perhaps?