I dunno. I think a casting director may be willing to make an exception for someone like Marsden. If Daniel Radcliffe (5'5"), Matthew Broderick (5'8"), and Darren Kriss (5'7") can do it (albeit, they're probably slightly bigger box office draws), I think Marsden (who is a much better suited to musical theatre than those guys) would be fine.
And I think Joel Gray was never cast as a traditional romantic lead because... he isn't a traditional romantic lead, in addition to his height (5'5") which would make him difficult to cast opposite many stage actresses. He's a wonderful performer, but he doesn't exactly read particularly masculine or conventionally attractive, and his voice is not suited to much of the standard baritone material. It would not be too difficult to find a leading lady to play opposite Marsden at 5'8"ish.
Updated On: 6/28/14 at 09:29 PM