The Oscar-winning filmmaker teased details of Javier Bardem's character in this month's issue of Empire.
On Bardem's performance, Mendes said: "It is larger than life in one respect but full of surprises - never entirely, at any given moment, exactly what you think it is.
"Javier takes some risks. But he's an amazing actor and I think he gives you something you may consider to have been absent from Bond movies for a long time."
The director also explained that the character deserved such a notable star to play the role.
"Silva's up there with the greats for me, and that's why I worked very hard to persuade one of the great actors of the world to do it," he revealed.
He also claimed that the upcoming Bond film, the first since 2008's Quantum of Solace, has "quite a lot of new things".
The full interview appears in this month's Empire magazine, on sale today (Thursday).
Skyfall will be released on October 26 in the UK and November 9 in the US. Watch a trailer below:
James Bond - Skyfall: photo gallery