The BBC sci-fi showrunner told press at the official Doctor Who Celebration at London's ExCel that Capaldi "leaps around the place" in "post-regeneration madness".
"We've only seen the tiniest moment - it's just the beginning of a process," Moffat explained.
"But of course, he's brilliant and... he's an extraordinarily vital and physical performer."
Doctor Who's head writer went on to insist that 55-year-old Capaldi's portrayal of the Doctor is "not at all elderly in style".
"People keep talking about this 'elderly' Doctor that we've cast… he leaps around the place probably more than any other Doctor in that first scene," Moffat revealed.
"But that's not how he's going to play it - that's just him in the post-regeneration madness. It's something we're still working on."
Capaldi made a cameo in Saturday night's 50th anniversary episode, but will make his official debut in Doctor Who's 2013 Christmas special.
"It is a proper finale to Matt Smith," Moffat said of the festive episode. "It's the story that we've been telling since he first put the bow tie on - that's what we're going to resolve.
"A lot of stuff we've left hanging we tie up there. And it's Trenzalore!"
> Matt Smith's Doctor Who exit was brutal, says Jenna Coleman
Watch a teaser for Doctor Who's Christmas special below: