Watching Don (2006), it's actually noticable how much of mannerisms Don actually showed when he was posing as Vijay posing as Don.
You could see his behaving like him, speaking like him, acting like him and many other details.
"For example if you look closely, Don when went to visit the school of Deepu, he did not recognized him at first and was surprised when he came to him, only when he calls him Vijay chacha, he gets hold of tge situation. And then him being clever as he is, managed the situation excellently".
"Also he was about to tell truth to Roma when thry faught but Boman Irani arrived just at time" and many more like these.
It's fun finding clues which were all over the movie.
Had it not been the remake, maybe many eagle eyes viewers or even those who watch movies closely could have called the bluff but it being a remake caught everyone off-guard with the ending.