It's not just a matter of the actor repeating some physical tics (and anyway, I've seen Tennant in one or two other things and, while he many not have the greatest range in recorded history, he's certainly got more than one character in him). When Tennant reads his lines, you can see him wearing a pin-striped suit instead of gaudy, quasi-period dress. And those lines are thoroughly Doctor-ish:
Casanova: I'm a spy.In Davies' hands, Casanova is a thoroughly good man whose default mental state is an almost overwhelming selfishness. A man with the attention span of a hummingbird who is capable of a terrifying single-mindedness ("Are you a magician?" Casanova is asked by a fellow prisoner when an unjust accusation lands him in prison. He thinks for a beat. "I can be"). A seductive innocent. Sound familiar? (Honestly, just see for yourselves.)
Grimani: A spy?
Casanova: Yes, that's right, a spy. Of course, being a spy I shouldn't say I'm a spy or I could get spied by a spy.
Grimani: I suppose you can prove it?
Casanova: What? You want me to spy on something? Look, there's a canal, I spied it. Look, it's still there. Look, and again.
In many ways, this actually clears up a lot of the issues with Tennant's performance as the tenth Doctor (while, obviously, raising other issues with Davies' writing of said character). No wonder the two best episodes in Doctor Who's second season were "The Girl in the Fireplace" and "School Reunion," which both repositioned the Doctor as an interstellar lothario, engaged in an endless sequence of deeply meaningful romances with women whom he eventually abandons. Unfortunately, this characterization isn't a perfect fit, and whereas Casanova can be summed up as a pleasure-seeker who always wants more than he can get, the Doctor is--or should be--a little more complicated. The result, as I've noted elsewhere, is a character without a heart. Now we know why.