They need to be young and skinny and it would help if they could sing. It would be better if they were american (less time with dialect coach) but that's not a big deal.
Here's George Blagden, aged 23, good-looking and both an actor and singer:
[ame]http://www.youtube.com/watch?v=w30Irt3m_jU[/ame]
How about Aaron Tveit as Bernie?