He can't is the short answer! In fact he seems to know even less that we do. Although in this excerpt from his interview with The New York Times(via Cinematical), he does reveal that he will be involved in at least some form of action shots for the film..
NYT: In “Spider-Man” you’re playing a character not that many people have heard of. What can you tell us about Van Adder?
Khan: Is that his name? These science-fiction names don’t mean a lot to me. He’s a character who, I think, was not in the comics or perhaps a minor character. I’ve read some pages, but the whole script hasn’t been made available to me.
NYT: How did you get the role?
Khan: My manager told me that the director Marc Webb was approaching us. I spoke with him, and he sounded very engrossed, and I was interested. I’m expectant and really looking forward to it.
NYT: Have you had to do any training in preparation for the part? And what about being in 3-D?
Khan: You don’t do training for action. You’re waiting for action. You just strap yourself into a harness and go. Two dimensions, three dimensions — “Avatar” really made a difference in terms of 3-D, but beyond that I haven’t thought about it.
Nels Van Adder is indeed a fairly minor character from Spidey lore. He was involved with Oscorp, and was a test subject of sorts for Norman Osbourne's serum which turned him into The Proto-Goblin. A demon like creature with fangs, claws, and impenetrable skin. Its obviously unclear if this is the route the character will be taken in Webb's movie, but since Van Adder is involved in some capacity, its a safe bet Oscorp..and indeed Norman Osbourne will be too.