Rory was first introduced in season five opening 'The Eleventh Hour' as a minor character, but has since developed into a full-fledged companion of the Doctor.
Darvill explained that while the two characters were initially at odds over Karen Gillan's Amy Pond, they have now developed a close friendship.
"I think Rory and the Doctor's relationship has grown so much over the last season," Darvill told Newsarama. "They've got a real bond, they've got a real kind of true friendship.
"I think they've got, you know, a very, very strong relationship, and as much as Rory can be a bumbling idiot at times, I think he's proved to the Doctor and to everyone else around that, when everything starts kicking off [he] can really step up to the plate and deliver."
The actor described the second half of season six as some of the strongest work the three actors have done and promised an 'epic' season finale.
Darvill also commented on whether or not future Doctor Who episodes could feature companions from the past, such as Rose Tyler or Martha Jones.
"I mean Steven [Moffat] is very good at [kind of] keeping it really separate, but I wouldn't put it past him to put us all together," Darvill said. "I mean maybe it would be some kind of strange self-help group to get over [it] and everything with the Doctor. Yes maybe that would be a good episode, just all of the past companions in therapy."
Doctor Who continues on Saturday on BBC One.
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Watch our recent interview with Doctor Who star Arthur Darvill below: