Curtis, who wrote Saturday's episode of the show, said that he likes watching the dynamic between the pair's characters.
"I love them, I love them," he told DS. "I'm really interested in the relationship in all manifestations of those two.
"And I think they're a very good one, which is just the very complex character of a 900-year-old non-man - still excited, still youthful, knowing everything, all of that - mixed with just a completely normal girl. I think they're a very interesting version of that and I love how sort of modern and upfront she is. And it means he isn't as romantic as some of the others have been. I find it gripping, the two of them."
He added: "I hope they'll let me write their marriage episode."
The episode of Doctor Who directed by Curtis airs on Saturday at 6.40pm on BBC One.