Matt Smith & Karen Gillan

Doctor Who, BBC's cult science-fiction drama about a time-travelling humanoid alien known as the Doctor, is 48-years-old.

Matt Smith, the Doctor's latest physical incarnation, is 28. And a youthful 28 at that. In person, the one-time English football prospect - he's a supporter of Blackburn Rovers - is highenergy and affable, with a tendency to bounce up and down with impulsive excitement, as though, like the character he plays on television, each thought comes hurtling at him out of the blue.

He's tall, too. The TV screen often masks actors' real height. Smith's jouer la comédie partner in Doctor Who,...