4.
In Mary Poppins, the kids playing Jane and Michael had no idea that Mary’s carpet bag had any ~magical~ properties, so when she pulled out an entire coat rack from it, their faces of shock and disbelief were completely authentic.
10.
To prepare for Saving Private Ryan, all the main cast members were sent to a harsh 10-day boot camp, except for Matt Damon, who stayed in America. Matt’s character was resented by everyone, and he was purposefully excluded so that the on-screen hostility would be as realistic as possible.
13.
Jay Baruchel was so terrified of rollercoasters that he made a deal with Knocked Up director Judd Apatow – he would appear naked in the earthquake scene as long as he didn’t have to get on any of the rides. If you’ve seen the film, you’ll know that Judd went back on the deal.
15.
The first dance of the Yule Ball is one of Harry’s more awkward moments in the Goblet of Fire, and that was no accident – Daniel Radcliffe was given way less time to learn the choreography than the rest of the cast were.
16.
This one’s more a case of nature catching the cast off guard, but it’s worth a mention. In Call Me By Your Name, there’s a scene where Elio and Oliver are climbing up a hill to the waterfall. It’s a happy scene, but the actors’ whoops are actually a reaction to the stinging nettles that covered the field.
17.
Avoiding leaks in a Marvel movie is basically an extreme sport, so for Endgame, the directors weren’t messing around. When it came to shooting Tony’s funeral, certain cast members were under the impression that they were part of a wedding scene.
Source: https://www.buzzfeed.com/hanifahrahman/actors-directors-caught-off-guard