Bokeem Woodbine is a recognizable actor who has been acting for some time and was last seen in the remake of Total Recall that came out this year.
This is what The Host Wiki Page says about Nate:
Humans are the current inhabitants of Earth in general, and the targets of Souls. Most humans have already been assimilated by the souls, but a few “wild” humans remain. They are considered “insurgents”. Some souls are able to get along with humans if they care enough to do so. Examples of this are Ian and Wanda, Kyle and Sunny, and Burns and Nate.
There are no specific details on Nate on the Wiki. Since both Johanna and I both have lent our copies of The Host out, and Johanna has read the book more recently than I, I am relying on what Johanna told me. Nate is the leader of the other non-Soul-inhabited human group we meet at the end of the book. He and the group have a Soul-inhabited human with them named Burns (Burns Living Flower), with whom they get along with.
Nate may play a bigger part in the sequel, which may be why they went with someone with a good acting background.
Feel free to correct us on who Nate is in the book or provide more clarification. In any case, welcome Bokeem Woodbine to The Host! Almost three months until the film hits the theatre!