Spitelout the Viking
DT has only about 4 or 5 distinct lines in the film (although he may have more when lots of Vikings are speaking at the same time). His first line is very near the beginning of the film as one of various Vikings who tells Hiccup to get inside (to be safe from the dragon attack).
His character, Spitelout, is never named outloud in the film, however DT's Viking looks suspiciously like the child Viking, Snotlout. Since their names and their looks are so very similar I'm betting they are meant to be related, perhaps father and son (although this is never mentioned in the movie). David is listed in the credits as "Spitelout", so we do have confirmation that that's his character's name.
Here's a screencap of Spitelout (it's from one of the DreamWorks animator interviews, although he's in some of the trailers too)
He stands out a bit because of his lack of facial hair (most of the adult male Vikings have massive bears, but Spitelout just has stubble). Spitelout can usually be found standing close to Stoick (Hiccup's father, who is also the chief/leader). In the books, Snotlout's dad was Stoick's brother, which may still be the case in the movie or he may be Stoick's second-in-command. He's actually on screen quite a bit alongside Stoick, but he doesn't say very much. He is never named, nor is his relationship with Snotlout mentioned, so it's all just speculation on my part.
Toward the end of the movie, when Stoick is leading a raid against the dragons, only 4 characters approach/enter the cave. Two are the main adult leads (Stoick and Gobber), and the other two are DT's Viking/Spitelout and a female Viking (I'm guessing this is the one played by Ashley Jensen). DT's last line where I could really hear him was after the Vikings (mistakenly) think they have won -- DT shouts out something victorious (something like "We defeated them!" although I don't remember the exact line). During the big battle later Stoick has Spitelout lead the rest of the Vikings to safety as he goes back to attempt to help Hiccup (which is why I think he may be 2nd in command). He's at the very end of the movie too right behind Hiccup (and I think Toothless may jump on him in that same scene - I 'm not sure on that one though).
Overall the movie was quite enjoyable. The story is VERY different from the books. My eight year old son has been reading the books (he's up to #8), and when I asked his opinion afterward he said that he loved the movie, but he still thinks the books are better. Still, the animation was great, the 3D well done, and the story is at times heartwarming and at times very action-packed. Just don't go expecting to see the movie of the book, because it is almost completely different. The only similarities are 1) some character names, 2) there are dragons and Vikings, and 3) Hiccup is clever and not cut out to be a typical Viking.
Hopefully this helps someone. All of this is from one viewing in the theater, so hopefully I got it all right. Please go see the movie and decide for yourselves if I've picked the right Viking (although with a few of his lines it really seemed obvious to me).
This is also my first LiveJournal post, so please be gentle with me. :)