The characters here don't just solve ciphers; they are ciphers, their dialogue and emotional interactions no less mechanical than all the secret switches and hidden compartments (left for them to discover by long-deceased puzzlemeisters who clearly had too much time on their hands). Sole exception is Bruce Greenwood, exuding his usual gravitas as a fictive U.S. president -- an honorable figure who typifies the film's fuzzy nostalgia for simpler, more patriotic times.
Variety

What are Helen Mirren and Jon Voight doing soaked in a raging underground river? And why's multiple Oscar nominee Ed Harris snarling like a silent-screen baddie? How about debonair Bruce Greenwood, skulking through a cave? And what's with Harvey Keitel, as a G-man cartoonishly exclaiming, "the President's been what?!"
Philadelphia Inquirer

Did you ever think you could sneak into the Queen's private study or the president's Oval Office? Or that you could kidnap the president (played well by Canada's Bruce Greenwood) in hopes of cajoling out of him the location of a secret book containing the answer to every wild conspiracy theory? Or that Mount Rushmore might be more than just a heady tourist attraction?
Toronto Star

Cage is at his best here when he acts with Bruce Greenwood, who gives an unnamed current U.S. president some of the charisma of Greenwood's JFK in Thirteen Days (2000). Greenwood understands the hopefulness and humor of depicting a chief executive today as a man of curiosity and wit; everything he does has a sparkle to it.
Baltimore Sun

The only time this varies is during the above-mentioned kidnapping of the president so Ben can get at a clue in the President's Book, a legendary tome containing all the nation's paranoid secrets. Luckily, the president turns out to be nice old Bruce Greenwood, who played JFK in "Thirteen Days" so he already has more presence than any sitting president you can think of.
Boston Globe

Bruce Greenwood plays the U.S. president, who figures into the proceedings in a far-fetched way. Greenwood ought to just throw his hat into the ring for 2008 since he seems to be filmmakers' go-to guy for residence in the White House.
USA Today

Quiet and unassuming as it is amid the big stunts and car chases, a little adventure Cage and Greenwood share is one of the movie's most-satisfying scenes. Greenwood, who played John F. Kennedy in "Thirteen Days," has the aura of a president - or at least, the aura we'd like our presidents to project. You almost wish the movie would take its cue from "Air Force One" and let the commander in chief lead the action, rather than Cage.
Canadian Press

"National Treasure: Book of Secrets" would seem like more of a B movie if it wasn't filled with such a dream cast. You can watch every Oscar film this year, and you probably won't find a better group of actors than Cage, Jon Voight, Helen Mirren, Ed Harris, Bruce Greenwood and Harvey Keitel.
San Francisco Chronicle

The attempts at comic relief fall flat mostly - in fact, it's probably a sad testament to the state of our country that the only laughs are uncomfortable ones involving the movie's starry-eyed view of the presidency. After discovering that one of the former commanders-in-chief unlocked one of the puzzles himself, Voight's character shrugs matter-of-factly: "Of course they did. They were the president, the smartest, most capable man in the country." You can probably imagine the tittering that went through the audience. Add to this the presence of Bruce Greenwood in the role of the current president - portrayed as man exuding honor, competence, and charm - and it's, well, kind of depressing, to be honest.
Premiere