Doctor Who: 13 Cool Details In David Tennant & John Barrowman's Last Of The Time Lords Commentary
9. John Barrowman's Hilarious Reaction To The Face Of Boe Twist
While the reveal that Jack is the Face Of Boe doesn't occur until the end of the episode, the three actors actually start discussing it around 15 minutes into the commentary, with Barrowman telling a funny little story about the first time he read the script.
He recounts that Tennant once showed up at his trailer and told him that he had to read it right away, because there's some sort of big revelation at the end.
Unfortunately, Barrowman didn't get the chance to read it until two days later. But when he did, he knocked on Tennant's trailer, and when the door opened, he let out a huge roar of "I CAN'T BELIEVE IT", which, by our estimation, is a perfectly natural reaction to discovering that your character will one day turn into a massive head.
This also indicates that showrunner Russell T Davies didn't tell Barrowman about the twist beforehand, which is actually quite surprising.