Without giving away too much: in between playing as Ezio and discovering more of Altair, modern-day Desmond Miles is in an 'Animus dreamland' with "strange platforming missions."
These first-person moments are limited to Desmond and won't appear elsewhere in Revelations. Ubisoft are just full of surprises considering they've got some tower defence play in there too.
Desmond's Animus levels "take the form of strange platforming missions," revealed mission design director Falco Poiker. "You'll have to play it to understand, because it's very hard for me to describe it, but it's kind of like a Dali-esque first-person platformer." Why Desmond is 'stuck' in the Animus remains at the end of Assassin's Creed: Brotherhood.
"He's trapped in his own mind. Our best comparison is Inception - Desmond is in a dreamland, but is able to access Ezio's memories. Ezio in turn then goes and accesses Altaïr's memories." In Brotherhood, Desmond already had some platforming fun when you unlocked the puzzles left behind by Subject 16.
"There's a character you'll meet in the Animus that you'll be able to talk to... there's been hints about this character before. I don't know if you'll be able to determine who that character might be from that but..." Ubisoft has kept any real specifics of Desmond's time in the Animus under tight lock and key, not wanting to spoil any surprises coming our way.
Assassin's Creed: Revelations releases on Xbox 360 and PS3 November 15th, and on PC the 29th.