Quote from the Herald Sun:
He described the role as a cameo. "It's just a great character..."
Unless it refers to Bana's own fame as an actor, a "cameo" usually applies to a prominent character that an audience has seen previously, even if that character was unnamed earlier.
In the context of a time-travel story with Romulans, Bana could be the commander of that Warbird in TOS "Balance of Terror". This would be an attractive supporting role for an actor who's a longtime Star Trek fan.

Mark Lenard (left) as the Romulan commander in "Balance of Terror"
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TOS Communicator Replica
Is anyone else thinking that this might be Future Guy?
Just a thought. Most people seemed to agree that Future Guy had a Romulan-esque silhouette and he seemed very interested in altering Federation history.
I know Enterprise isn't nearly well known enough to make this a major plot point, but if he was introduced as a new character and shown just once using a temporal communicator it would wrap up a niggling old plot hole.
A cameo would fit his modus operandi, not travelling himself but using temporal communicators to enlist the aide of other villains.
If this is true, I think this is a good thing. Men are their own worst enemy much of the time. I've never liked these movies focusing too much energy/time on baddies that we neither know well, nor have much invested in. You really can't do enough character development to actually feel too much about characters like Shinzon or even the Borg Queen.
Man against situations of fate...or situations of his own making...or against bigger issues. It's still a good/evil thing much of the time...its just that good/evil never plays really well in being represented in individuals. The Romulan society/values against the UFP's society/values is bigger than the players.
Of course, Kahn is kind of the anomaly in all this...that movie worked really well, but I blame that partially on already knowing the character from TOS.
All this to say...I'd be more than happy to see our baddie set things in motion...then disappear or meet his untimely demise and then let the rest of the movie deal with the consequences.
That's odd. The main villain is just a cameo? Good actor, though.
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"Thank you. I'm here all week. Try the veal!"
For some reason I didn't expect such a ringing endorsement from Bana. If he, and Greewood, and Nimoy are really that impressed with the script, then that's great news indeed. These are three actors who's talent and experience I greatly admire. Not to mention none of them really needed to take a chance by accepting a role in the eleventh Star Trek movie. Since they have, AND they're openly praising the movie, that goes a long way towards increasing my optimism.
I'm actually glad he's saying his part is small. Maybe we won't get such a two dimensional story where the focus is entirely "bad guy vs. good guy/fate of Earth hangs in balance".
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"...I don't know. It's going to take a long time, and a lot of soul-searching before I find the answer. But right now we have a mission to complete."
-Benjamin Sisko, Blaze of Glory
I love the idea that everyone is really on board with this movie, no matter how "big" the part. Seems the only real ego about it all is Shatner, and even that's only to an extent.
Here's to hope though that since this movie seems so complex with so many storylines and plots, they don't cut it short. In my opinion that's what ruined Nemesis, cutting all the meat to make a more action oriented film. Make a good movie J.J., don't worry about length, just make it good.
If it's 2 and a half hours and has a lot of good substance to back it up, should be one helluva ride!
JB
I wonder if this means Bana's character is being set up for a return in a future film, then.
Al