Let me lead this off with the following: I would be incredibly surprised if The Hulk has any role in Captain America: Civil War. Considering what I know about the story of the film it just doesn't make sense, and there really isn't room for him. I mean, anything is possible, but man, this movie will be packed if they put Hulk in it.
Does that mean Bruce Banner isn't in it? I don't know. Again, I don't know where he might fit, but anything is possible. And now Mark Ruffalo is saying that he will be in the movie... or at least that Robert Downey Jr told him he's in the movie.
If Robert Downey, Jr. says I’m in Captain America, I’m in Captain America dammit! And I will wait for the day that my call sheet shows up at my doorstep or my script. At this point, I’m told that I’m in it by Robert, which I’ll take as biblical, but I just haven’t heard or seen from production yet.
This is less unusual than you might think - only the main leads in these movies ever know what is going on. Supporting actors do not get the entire script, and sometimes, when it comes to cameos, they don't even get any pages at all. Anthony Mackie has said that he didn't know The Falcon joined The Avengers until he saw the movie at the premiere - he just came in and shot a few hours of stuff and nobody told him what he was doing. Samuel L Jackson doesn't know what Nick Fury is up to anymore in the Marvel Cinematic Universe, and he was as suprised as we were that he wasn't cast in Civil War.
That said, there are only six weeks left in the production of Civil War, and you would think that Marvel would at least give Ruffalo some dates when he would be needed so he could schedule things. He's a working actor, after all, and if believes his schedule is clear he could go ahead and book a role in China for all Marvel knows.
It would be interesting if Tony Stark knows where Bruce Banner is a year after he disappears at the end of Age of Ultron, and if he is helping the scientist stay on the lam while also using him for his science bro abilities. That sort of cameo - Tony and Bruce talking on Skype - would be easy for Marvel to squeeze into Ruffalo's schedule. Also, all Marvel movies do planned reshoots - could they have a Banner scene penciled in for the reshoots?
Again, I would be shocked to discover that The Hulk has any real role in Civil War, but I've learned to expect shocks from Marvel.