"Cuaron, who recently sat down with THR's Stephen Galloway, provided some info on the departure. "I thought I had written a small movie ... just one character floating in space," Cuaron explained. "We started developing stuff [trying] to figure out the technology. And the luxury [was] that we could try many things. And part of that was conversations with actors. I had conversations with Angelina, but then she went to do one film, and then she was going to direct ['Unbroken']. Something happens, you part ways."Of course none of which explains what happened later about Cuaron apparently wanting Natalie Portman but the studio all but forcing, thankfully because she's fabulous, Sandra Bullock on him.
As for RDJ, Cuaron didn't want to pen the energy of the actor into a film that wouldn't utilize his talents. "It became very clear that, as we started to nail the technology, or narrow the technology, that was going to be a big obstacle for his performance. I think Robert is fantastic if you give him the freedom to completely breathe and improvise and change stuff. [But] we tried one of these technologies and it was not compatible," Cuaron said. "And, after that, we [had a] week that we pretended as if nothing was happening and then we talked and said, 'This is not going to work. This is tough.'"
Film As an update to my rumour history of Gravity or whatever this was, here's a link to The Playlist in which Alfonso Cuaron somewhat explains what happened with Angelina Jolie and Robert Downey Jr during the casting process: