Although they have their own distinct personalities, the Tenth and Eleventh Doctors are very similar. They share a lot of the same quirks and in The Day Of The Doctor, they even end up finishing each others sentences. These similarities have left the Doctors personality in broadly the same area for just shy of nine years. But now, its all change.
Steven Moffat and Peter Capaldi have been at pains to emphasise how much darker the Twelfth Doctor will be with declarations like No frills. No scarf. No messing. Just 100% Rebel Time Lord and trailers that play up the new Doctors serious personality above everything else. While we wont know for sure what the Twelfth Doctor is really like until Deep Breath airs, well leave you with this quote from Steven Moffat that suggests the Twelfth Doctor will have a touch of the Sixth about him:
He has a tremendous ability with throwaway humor and a lot of it is around the fact that sometimes he is terribly rude. I think kids will think he is the rude Doctor [...] You might want to cuddle him but he really will resist.