They could use faces from the trailers. So don't be surprised if you see a default Hawke.
David already said they would not just use a default Hawke or Warden.
Quote from PaxEast 2012
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Q: Hypothetically, of course, in the hypothetical Dragon Age III, let's say we have a hypothetical cameo from say, the Hero of Ferelden. Hypothetically, would he or she be voice acted, or would he or she just show up and just kind of be there --
A David: Oh, you mean the Warden?
Q: Yes, the Warden.
A David: If the Warden showed up, would the Warden be voice acted? [David and Mike look at each other, laughs from audience] OK, if, if, if we... IF, if we put your Warden in the game we would have to take into account the fact that technology has changed, the look of the game has changed, we have voiced characters -- there are no silent characters in the game. I think ideally, we would want to throw that to you as a choice, how you would like that presented. [Looks at Mike] Is that fair?
A Mike: I think so, yeah. I think the under current of what Dave's saying is that we all kind of go whoo [exhale], is, to be quite frank, we'd have to do that so right. Because, again, it's your character, and that really is something that we'd want to put as much of that agency ON YOU, to own -- you know, how they would look and/or sound even -- as we could. And I think that would be the only way to make that character do a cameo. And the really the question is: is the cameo worth the potential disconnect? I don't know. I think it would really depend on the overall story.
A David: We'd wanna do it right, or not at all.
A Mike: Yeah. That'd be basically it.
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So there you are. It would be our choice. So the only way they would include them in a future title is to create a CC and let us Recreate our Warden and Hawke. That way it is our choice to how they would look and sound. Otherwise they won't do it.