James Franco isn't in Dawn of the Planet of the Apes... except maybe he is! It turns out he has a cameo in the film (according to Entertainment Weekly), but that comes as a surprise even to the actor. He gives the best quote ever:
“Wait, I have a cameo? I didn't know that. You know, I don't even think they asked my permission for that.”
According to EW his character of Will Rodman shows up posthumously, having been killed in either the Simian Flu or the wars that followed. Apparently he's on a video tape, possibly apologizing to the surviving members of humanity for his role in creating the end of civilization.
Of course nobody's quite sure. Franco shot some alternate ending stuff for Rise of the Planet of the Apes where he died, so maybe that's what's in the film. He did talk about making a cameo in the sequel waaaay back before Matt Reeves was hired:
“I was going to be a small part of the next one. There was one scene, between Caesar and my character, maybe even just like on a video that was left behind, but then a lot of things happened, like [former Fox co-chairman] Tom Rothman who was a big part of the first movie, left. Now Rupert's not a part of it so I don't know. My guess is I won't be in it. Nobody's talked to me since Rupert left.”
I do hope it's a video, because that's a nice callback to Battle For the Planet of the Apes. In that film Caesar takes an expedition to the atomic bombed ruins of a human city to find video footage of his parents, who traveled back in time from the distant future and set into motion the events that led to apes ruling the world (best time paradox in movie history). This film already feels like a remake of Battle - where a burgeoning ape civilization squares off against a declining human civilization - so I approve heartily.