The actor gave chat show host Ross clues about the BBC sci-fi drama's 50th anniversary special, in an episode to air tonight.
"I've got to be diplomatic... because if I give the whole game away I'll be screwed," Smith joked. "The thing is with Who, you'll understand this more when you've watched it."
He then added: "Paintings. That's all I'm saying."
The 30-year-old went on to say that he is "very happy" on Doctor Who and confirmed that he will shoot a 2013 Christmas episode following the 50th 3D special.
"I go and I do the anniversary special, then I go away for a bit and then I come back and do the Christmas special," he explained. "It's one of those jobs that you have to take year by year, it's ten months a year, it's all-consuming.
"So I don't think you can plan five or six years ahead, or even two years ahead. It's a year by year thing and at the moment it's 2013 and we'll see what 2014 holds."
The Jonathan Ross Show airs tonight (Saturday, March 16) at 9.20pm on ITV.
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Watch a trailer for Doctor Who's new episodes below: