He was the best thing in Zodiac, which is saying something, and the best thing in A Scanner Darkly, which isn't. He's Robert Downey Jr. and he's a blinkin' genius. Christopher Orr in The New Republic gets it just right:
Is there any actor alive who takes more obvious delight in his line readings than Robert Downey Jr.? He is precise yet baroque, contemplating each word with casual bemusement as it leaves his mouth. Though he is one of modern cinema's fastest talkers, it's not because he's in a hurry to tell us anything. Rather, he seems to feel that once a remark has passed through his mind, it's already happened; uttering it aloud is almost an afterthought. It's a form of delivery at once self-deprecating and self-absorbed: Are his thoughts unworthy of being shared? Or are we just unworthy to hear them?