Rachel McAdams' startled reaction in the broom closet was genuine, since the mop handle fell completely by accident, and scared McAdams almost out of her skin. Scott Derrickson decided to keep the reaction in the film, since she never broke character.
Mads Mikkelsen admitted that with all the computer-generated imagery he got a bit lost on how to film his scenes, especially during the Mirror Dimension scenes.
(at around 1h 28 mins) The scene where the Cloak of Levitation wipes away Strange's tears was improvised on set. It was a suggestion Benedict Cumberbatch made to the crew, who liked it enough to create it.
Chiwetel Ejiofor is a fifth-degree black belt in karate, so his movements are second nature in fighting.
According to the 2021 book 'The Story of Marvel Studios: The Making of the Marvel Cinematic Universe', Benedict Cumberbatch was originally offered the part of Malekith in Thor: The Dark World (2013). Despite being a fan of the MCU, he declined because the part did not thrill him enough, and he preferred to wait for "something a bit more juicy." He was later Marvel president Kevin Feige's first choice for the lead role of Stephen Strange, but he initially had to decline the offer due to his theater commitments playing William Shakespeare's Hamlet, which clashed with the film's proposed shooting date. Joaquin Phoenix was the studio's second choice for the lead role, but after a few months of deliberation, he eventually turned down the offer due to his reluctance to do a multi-picture deal that was required by the studio if he signed on. When the film's release date was pushed back from July 2016 to November 2016, Cumberbatch was finally able to commit to shooting.
Stan Lee: (at around 1h 18 mins) man on the bus reading "The Doors Of Perception" by Aldous Huxley. The book is a recounting of Huxley's experience taking psychedelic drugs, with particular mention of their use in perceiving art. Dr Strange comics have long been associated with psychedelic drugs because of their use of surreal images to portray magical dimensions and beings. Marvel has always insisted that drugs played no part in the creation of Dr Strange, which may be why Lee is laughing at the book.