Straight in a straightforward, un-ironic sort of way. As Jack Harkness, he brings irony (though to my mind, not nearly enough), but I think as Carrot, he could tone that down. The tricky thing about Carrot is that ambiguity; is he really that... straight? I think Harkness could totally convey that. And I can't think of any other actor who could strike that balance, they're all either too ironic or too innocent. Yes, the more I think about it, Captain Jack is our man.

And he'd look totally hot swinging a sword.