Surprisingly, he's not all that raspy, but his voice is very powerful. The Burned Man wasn't at all what I was expecting him to be after I first heard about him in New Vegas, but he's turned out to be even better than I would have imagined. There's a beautiful mixture of underlying insanity and straightforward, "this is how shit is going to go down, deal with it" motivation. The bible zealotry you saw in the trailer appears to define his character, but the verses he quotes aren't coming from the "love everyone" bullshit that's found in most of the bible.
Joshua Graham was truly worthy of serving the mighty Caesar, even more so than legate Lanius.