It turns out that it wasn't just the scenes of Iron Man in his suit in "Iron Man 3" that didn't include Robert Downey Jr. actually acting out the scene.
As previously reported, Downey was forced to take six weeks off during production to recover from an ankle injury. That kind of delay would've destroyed the studio's plans to get the film into theaters by May 3rd.
Instead, a special effects team was brought in to continue filming. Essentially a body double shot the scenes live action, while Downey's face was shot acting out the scenes and digitally inserted over the double.
Visual effects supervisor Chris Townsend says: "We were able to reconstruct [Downey] as Tony Stark on set, with the help of a body double and the facial captures we collected afterwards."
Similar techniques were used in films such as the early scenes of Chris Evans in "Captain America: The First Avenger," and the way Armie Hammer became his own twin in "The Social Network".
The entire beach house/cliff scene at the end of "Iron Man 3" was reportedly constructed using this technique.