The actress, who will depart the sci-fi drama midway through its seventh series, previously declared that she will not return for cameo appearances.
"If I do something, I want to just do it," she told Collider. "I just feel that it's a stronger impact, and I like to go with my instincts because it's more exciting."
Gillan added that she did not want to know the circumstances of her character Amy's exit until it was absolutely necessary.
"I wanted to keep it a surprise, for as long as possible," she explained. "[To help] keep the excitement in there."
The Scottish star also praised Doctor Who showrunner Steven Moffat for writing such a "strong female" in Amy.
"A lot of stuff is clearly written from a male perspective and the women aren't quite as fleshed out as the guys," she suggested. "Whereas this character is really well-written. I just hope I get to do more of that."
Gillan previously promised that she will leave Who "on a high" in a "damn good" storyline.
Doctor Who will return to BBC One in the autumn.
> Doctor Who writer Toby Whithouse: 'I know about the new companion'
Watch Karen Gillan discuss her Doctor Who exit below: