In an online blog post, Millar wrote: "Nothing seen in this picture wasn't in the screenplay 18 months ago. Yes, the body-count is very high, but a movie called Kick-Ass 2 really has to do what it says on the tin.
"A sequel to the picture that gave us Hit-Girl was always going to have some blood on the floor and this should have been no shock to a guy who enjoyed the first movie so much."
Carrey apparently loved the first film in the Kick-Ass series so much, he turned up on Conan O'Brien's talk show dressed as the titular hero in 2010 after seeing the film.
Millar went on to call Carrey's work in Kick-Ass 2, in which he plays Colonel Stars and Stripes, "magnificent".
He called it one of "Carrey's best-ever performances".
"He's never done anything like this before and even from the trailer, with his masked dog sidekick specially trained to munch criminal balls, you can see that something really fun and special is happening here.
"Ironically, Jim's character in Kick-Ass 2 is a Born-Again Christian and the big deal we made of the fact that he refuses to fire a gun is something he told us attracted him to the role in the first place."
The film, which also stars Chloe Grace Moretz, 16, and British star Aaron Taylor-Johnson, tells the story of costumed high-school hero Kick-Ass, who joins a group of normal citizens inspired to fight crime in costume.
- Bang! Showbiz with nzherald.co.nz