Sir Ian McKellen is well-known for his portrayal as the mystical Gandalf the Grey in the "The Lord of the Rings" and "Hobbit" movies -- but when he was offered the role of Professor Dumbledore in the "Harry Potter" films, McKellen declined the chance to add another powerful wizard to his filmography. Now, fans know why.

In an interview with BBC News HARDtalk (via The Wrap), McKellen explained his reason for passing; he knew Richard Harris, who portrayed Dumbledore in the first two films, did not approve of him as an actor. After the death of Harris in 2002, Michael Gambon would play the headmaster of Hogwarts for the rest of the series.

RELATED: Sir Ian McKellen to Play Gandalf One More Time 

Interviewer Stephen Sackur recalls a quote from Harris, in which he referred to McKellen along with fellow actors Kenneth Branagh and Derek Jacobi as "technically brilliant but passionless."

"When they called me up and said would I be interested in being in the Harry Potter films, they didn't say in what part," McKellen said. "I worked out what they were thinking, and I couldn't...take over the part from an actor who I'd known didn't approve of me."

RELATED: Ian McKellan Jokingly Explains Why He's Not In Logan 

"You mean you could've been Dumbledore?" Sackur asked.

After a brief pause, McKellen replied, "Well, sometimes, when I see the posters of Mike Gambon, the actor who gloriously plays Dumbledore, I think sometimes it is me."