I saw "My Week with Marilyn" last night as well, and all I could think the whole time was how perfect he'd be as Marius if he could sing it (and, now that I've caught up with this thread, he clearly can). I also don't understand the logic that Marius must (or even should) be a heartthrob. He should be a young student (and when he said he was 23 in "Marilyn", I never doubted it) who sees a girl on the street, falls in love with her, and rather obsessively tries to find her. I would encourage anyone doubting this casting to go see "Marilyn" and watch how Redmayne excels at longing romantic looks, which comprises a decent amount of the characterization Marius has to embody. Plus, I thought it was a terrific film.
"Art, in itself, is an attempt to bring order out of chaos."-Stephen Sondheim