He goes cross-eyed when he casts magic, because otherwise we might accidentally respect him here.
The magic works: the spiders are driven away, and Sebastian returns to life. Radagast asks a bird where the spiders came from; it twitters something about "the old fortress." On his rabbit-pulled sled, Radagast follows the bird to Dol Guldur.
The Book: No such scene occurs.
What difference does it make? At this point in the movie, none of Radagast's activities affect the storyline of The Hobbit for better or worse. In fact, this scene fits in with the same "dwarves telling stories" plot device that allowed the Battle of Azanulbizar to be shown. Using a conversation between Gandalf and Bilbo as a framing device helps show this scene without it having any great impact on the story.
My Opinion: Every time the movie comes to this scene, I have to do a quick calculus in my head. How long is Radagast going to be on screen? How long would it take me to get up, find the controller, and change it to the next scene? By the time I decide "fuck it, I don't care if I lose any time, I hate this stupid character," he's already riding off on his sled of rabbits. Everything about Radagast annoys me. I hate his silly hat, the birdshit in his hair, and his stupid pointy Santa's-Little-Helper shoes.
Somebody actually made the intentional, deliberate decision to include these fucking things in his wardrobe design. They did this on purpose.
His mannerisms are bothersome; everything from the little noises he always makes to the odd facial twitches. He's a CGI body and racist accent away from being Jar Jar Binks. How anybody could have decided that this was a good character to include is beyond me. The fact that they chose to make this cartoon character out of an Istari is the worst character assassination in Lord of the Rings since Faramir. Radagast is not only terrible, but he completely changes the tone of the movie. The Hobbit is supposed to be a more childish and light-hearted story than the trilogy, sure, but Radagast's scene here is nothing but outright camp.
My hatred of Radagast aside, this scene is not entirely out of place, though it does have a few problems associated with it (since when can wizards bring things back from the dead? Was that vial a one-time use? Does it work on people? Why did Radagast get so upset over one hedgehog, but ignores all those other dead animals?). Radagast's home is in the Mirkwood, and it stands to reason that he would have had some issues with the giant spiders. So it's kind of interesting to see that addressed. And... that's it. I'm out of good things to say about this scene. Fuck Radagast.