One of the things I like about the silliness & craziness of the later incarnations of the Doctor in Doctor Who is that when he suddenly turn serious, angry or threatening, it carries VASTLY more weight. You know he’s serious when events manage to crack his child-like persona. Calling this a personality trait of the later Doctors though is a little untrue, as the Second Doctor played by Patrick Troughton was the first to start to exhibit those childlike personality quirks.
Anyway, one of my favorite examples of the crazy Doctor suddenly turning deadly serious is in the episode The Doctor’s Wife featuring Matt Smith as the Eleventh Doctor. After his usual crazy diatribes after thinking he has discovered a bunch of surviving fellow Time Lords, he discovers the distress messages of those now deceased Time Lords. Unarmed, he turns on those who have deceived him and in a suddenly slow, measured, calm and low voice:
“You gave me hope and then you took it away. That’s enough to make anyone dangerous. God knows what it’ll do to me. Basically… RUN!” Doctor Who – Season 6, Episode 7, The Doctor’s Wife
His would be captors do just that, turning and running from him in fear.
Another awesome quote from this episode is later, when The House has possessed the TARDIS:
The House: “Fear me. I’ve killed hundreds of Time Lords.”
The Doctor: (In a quiet & serious voice.) “Fear me. I’ve killed all of them.”