In the one-person / one shot FX production model (basically TV or low-budget model), the box analogy works fine.   Truly, adding a second day on the afternoon of the first day accomplishes little.

But the analogy doesn't scale up to the feature factory world, or to adding a second year to the first year.

In feature production (at least in Star Wars scale productions) nobody's building boxes - everybody's sanding their corner of the piece of wood that comes thru the slot every morning, and then passing it down the assembly line.   The "box" was designed long before you got your hands on it.   One year or two, doesn't matter.  You just sand the parts until the boss says to stop.   

Adding time is no biggie in this scenario (assuming the pay also scales proportionately o'course). 

Now you can sand your pieces of wood a little more carefully, or go home a little earlier...or keep working like a fiend because the total box order has been doubled along with the time.   But that's really all that will change.

But then, I type capital letters by toggling caps lock on and off

Andy Serkis, Domhnall Gleeson, and Max von Sydow were nice surprises. 

Episode VII Cast Announced

The Star Wars team is thrilled to announce the cast of Star Wars: Episode VII.

Actors John Boyega, Daisy Ridley, Adam Driver, Oscar Isaac, Andy Serkis, Domhnall Gleeson, and Max von Sydow will join the original stars of the saga, Harrison Ford, Carrie Fisher, Mark Hamill, Anthony Daniels, Peter Mayhew, and Kenny Baker in the new film.

Director J.J. Abrams says, "We are so excited to finally share the cast of Star Wars: Episode VII. It is both thrilling and surreal to watch the beloved original cast and these brilliant new performers come together to bring this world to life, once again. We start shooting in a couple of weeks, and everyone is doing their best to make the fans proud."

Star Wars: Episode VII is being directed by J.J. Abrams from a screenplay by Lawrence Kasdan and Abrams. Kathleen Kennedy, J.J. Abrams, and Bryan Burk are producing, and John Williams returns as the composer. The movie opens worldwide on December 18, 2015.