theprequelsrule said:

I've seen most of his films and he always seems to play his different characters in a very similar manner. Like Natalie Portman.

Most of his roles are him playing Han Solo/Indiana Jones, just in different circumstances - he tends to play his characters either as a slightly nicer or meaner version of them, depending on the film. Since a lot of people disagree with me this might be just my perspective.

But his performance in the Mosquito Coast (which I finally got around to seeing a few years ago) stood out as being very different: facial expressions, vocal tone, everything.

According to this review of Cowboys & Aliens, Harrison Ford "is certainly one of the few actors who can afford playing the same role over and over again and stay entertaining all the time." So, you are not the only one with such an opinion. I think in Witness he was a little bit different than usual too.