The film's cast are currently shooting in Chicago where locals watched a car chase and action sequence for the film, The Times of India reports.
"We saw his stunt double doing the action. He looked a little taller than Aamir and was dressed in black," said a source.
"It looked like he'd gone back to his Ghajini days. We had read reports of how Aamir wanted a new hairstyle for this film."
The source explained: "During the day, he hardly took his hat off. But when he did, we could understand what might have prompted him to keep his look hidden - it's that different."
"When not shooting or talking to the director, he was reading a book on Mahatma Gandhi. But he chooses to stay indoors. Only on Eid, there was a huge bash at his apartment," said the source.
Khan, who plays a negative role in the film, is awaiting the arrival of Katrina Kaif for the remaining action sequences.