For a while now, the character actress Maisie Williams will play in "Doctor Who" season 9 has tickled everyone's fancy. When she appeared in the trailer, fans could tell the character is rather spunky, calling the Time Lord "old man." 

Executive producer Steven Moffat served up details about Williams' role during the Television Critics Association's press tour. He emphasized that the "Game of Thrones" star will play someone that fans of the classic series aren't familiar with, making the mystery tougher to crack. 

"Once you see what she's up to you'll appreciate what a clever idea [the role] was. It's a significant role, we're not just throwing her away," Moffat said as quoted by Entertainment Weekly.

He then went on to describe how "terrific" Williams was in playing the "great part" for "Doctor Who" season 9. 

"One thing I think is worth saying, because it keeps coming up as an issue, is that she is not playing a returning character. She's a new character, she's not a character from the Doctor's past," he said, with Deadline quoting him as saying that "her role will develop in an unexpected way." 

According to Cinema Blend, the Doctor and Williams' mystery character are at least familiar with each other or have encountered each other in the past. In the trailer, the first time Williams' character sees the Time Lord, she says, "What took you so long, old man?" 

However, with Moffat emphasizing that the new character will technically meet the Doctor for the first time, it is difficult what to make out of that scene. Williams will appear in "Doctor Who" season 9 episode 5 "The Girl Who Died" and episode "The Woman Who Lived."  It was previously mentioned that she will "challenge" the protagonist in "unexpected ways."

"Doctor Who" season 9 airs Sept. 21.