My impression is that he was actually wearing fake teeth.  They did a good job of it, so that if I'd never seen Martin Freeman in anything else, I wouldn't have thought they were particularly odd.  But I believe they're bigger than his own teeth, perhaps intended to make him look "predatory."  The first few times I watched this movie, I had a bit of trouble looking directly at him -- like "what is wrong with this picture?"

But yes, he's wonderful in this role.  Looks like he was having a lot of fun with it!

And I agree, Russell, Nativity is also a lot of fun, especially with the tiny bit of singing and dancing that Mr. Freeman does.
 

Well, he's not playing John Watson, that's for sure!  So yes, this character's speech is different, but that may or may not have anything to do with those teeth -- same goes for Mr. Cumberbatch, since I believe he sometimes has a slight lisp in real life.
 

Yes, he does speak differently.  It's a different accent, but no lisp.  
I know Benedict sometimes has a bit of one is real life, but that's not nearly as much as in The Fifth Estate, and you can tell it's from the prosthetic.

Just watched this movie tonight. I didn't find the movie particularly good (although it had it's moments), but I watched it for Bill Nighy, Rubert Grint and of course Martin. Those three alone made the movie worth watching, I loved their characters.

Martin was incredibly creepy with his fake teeth and cold eyes. (And... probably a bit not good, but... darn was he hot!)

I want to see more villain!Martin, he was awesome!

Yes, I loved him in Fargo. Amazing acting.