The Scottish actor stressed that director Matthew Vaughn's film explores the darker side of the mutant's personality in order to paint a more accurate picture of the heroic Professor X.
"You give him an ego. You give him a sex drive. You make him a little more selfish. You give him a sense of humour," USA Today quotes him as saying. "You don't make him wise."
McAvoy also explained that joining X-Men: First Class proved to be the fulfilment of a childhood dream, adding that he has always been intrigued by the journey that leads Professor Xavier to become founder of the X-Men.
"I loved the cartoon when I was a kid - in the early '90s. My friend and I got really into it, actually. My favourite character wasn't Professor X. My favourite character was Gambit," he added.
"I met Matthew, the director, and he seemed quite cool and irreverent. I said, 'If we do this, can we take the character as far away as realised in the [previous three] movies?' There's no point in doing a movie if you're not going to show the transition."
Director Vaughn recently revealed that X-Men: First Class was influenced by the James Bond film series.
X-Men: First Class is now playing in the UK and opens on June 3 in the US.
> Watch our interview with X-Men: First Class star Kevin Bacon
> X-Men: First Class in pictures
> X-Men: Top 5 moments from the superhero saga
Watch the X-Men: First Class training montage below: