Gravity director Alfonso Cuaron has revealed why Angelina Jolie and Robert Downey Jr exited Gravity, being replaced by Sandra Bullock and George Clooney.

Both Jolie and Downey Jr were in line to star in the hit film but Jolie was unable to take part due to her busy schedule starring in Maleficent and directing Unbroken.

Downey Jr on the other hand is said to have struggled with the technology being used. Cuaron explained to The Hollywood Reporter, "It became clear that, as we started to nail the technology, or narrow the technology, that was going to be a big obstacle for his [Downey Jr] 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. 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'."

Gravity is nominated for ten Oscars at this year's ceremony including Best Film, Best Director and Best Actress for Sandra Bullock.