Plec's colleague Kevin Williamson previously revealed that he searched through hundreds of auditions before Morgan landed the role.
Plec has now told Entertainment Weekly that Morgan has read all of L.J. Smith's Vampire Diaries novels and has further researched vampire folklore.
She also conceded that she was impressed with Morgan's ability to show different sides of a character.
"He has a depth and an intelligence as an actor, but he's also got this great wicked grin and these dimples that make him seem both fun and naughty at the same time, even as he's being evil or very stern," she said. "So there's a playfulness about him that Kevin and I really liked, because we didn't want Klaus to be a one-note bad guy."
Plec continued: "The more layers that we can bring to the character, the more successful the character's going to be, and Joseph seems to have the ability to do all that."
Vampire Diaries star Candice Accola recently revealed that Klaus will play "tricks" when he arrives in Mystic Falls.
The show airs on Thursdays at 8/7c on The CW in the US and returns in the UK on Tuesday at 9pm on ITV2.
> Ooh, a peek at Klaus on 'Vampire Diaries'!