Sir Ian McKellen is setting himself up for a very busy year as he has signed on to star in a play called "The Syndicate" at the Chichester Festival Theatre in the U.K. The play will run from July 21 to August 20th. This also happens while The Hobbit will be in full production. Normally when an actor is signed onto a film they are unable to work on anything else partly because of cost and partly because of production needs. However, the print edition of The Dominion Post reports that the actor negotiates an "escape clause" in his contract so he could take on the role. McKellen is the second actor on the project that will get a middle of production break as Martin Freeman (Bilbo) must also take a break so he can complete work on Sherlock for the BBC.
A large reason that actors can come and go as needed is because both films are going to be done back to back and depending on how close the movies stay to the book there are actually large sections in the story where their characters are not involved allowing for a much more flexible schedule then normal for the lead actors. This isn't to say I wouldn't want to be schedule coordinator on the project as they must currently have many headaches trying to work all the details out but this flexibility is a key reason while Freeman and McKellen were able to sign on to the movies.