The actor takes on the part of Dr Curt Connors in the reboot of the Marvel superhero franchise.
He told Cinema Blend: "The majority of the time I'm me, I'm a human, not a Lizard. That's hard enough. I did have the CGI suit for certain scenes.
"The Lizard is nine-feet tall. There was one day that I went on set and they had a double, a big, big guy, playing me, as the Lizard. I looked at it and I went, 'No'.
"Following that, [director] Marc [Webb] let me know all the physicality you'll see in the Lizard is my own."
Ifans added: "I had a green suit on, and then this cardboard head and these big claws. It was the most f**king insane.
"Each and every time you see the Lizard, the technology is so advanced now that when the Lizard's eyes move, they're my eyes. If I frown or show any emotion, they're my emotions."
The Welsh star recently said that he was drawn to the role because of the character's complexity, claiming that he is not a traditional villain.
Andrew Garfield recently revealed that he studied spiders in order to copy their movements for the role of Peter Parker.
View the latest Amazing Spider-Man pictures below: