When Collin stepped out of the TARDIS, he saw he was at hogwarts. Then as he turned around the TARDIS was leaving! 'Oh Crap what if this is the past?' he thought. Then he saw an older version of harry walking on the grounds. 'Well, not the past, but dang that TARDIS, this is worse than weeping angels, at least then you don't have to worry about getting back!' (AN : weeping angels touch you to send you back in time) So, collin just decided to go up to Harry, 'he is the only one I know here' Collin tapped Harry's shoulder. Harry jumped.
Imagine that, Harry, hero, jumping at a shoulder tap?