Character Analysis
The Scarecrow's really just a cameo here—he got his chance in the spotlight in Batman Begins—but we get a little reminder of what he can do at the very beginning of the whole thing. In the comics, Dr. Jonathan Crane was a psychologist who enjoyed studying the effects of fear, usually by putting on a truly hideous mask and spraying his victims with a panic-inducing drug.
He's still at it here, peddling his fear gas as a drug and finally getting sent back to Arkham Asylum as the movie opens. It tidies up a bit of plot from Batman Begins—which ended with Crane still on the loose—and further emphasizes that the Caped Crusader is seriously kicking butt. With Crane going back to jail, it seems like Gotham can finally sleep easy at night.
Crane also serves as another connection between The Dark Knight and earlier incarnations of Batman. The Scarecrow is one of the most famous members of Batman's rogues' gallery, having first appeared way back in 1941 and created by Bob Kane and Bill Finger just like old Bats himself. He's shown up in other media too: notably in old cartoons like The Superfriends and newer ones like Batman: The Animated Series, and in the show Gotham where we get to see him as a very troubled teenager. Murphy is here to hold up that end, and quietly remind us that there's a whole lot more bad guys out there than the Joker himself.