Alfonso Cuarón, the director of Gravity, has explained why George Clooney was cast in the sci-fi picture instead of Robert Downey Jr.

Originally, Downey Jr. and Angelina Jolie were set to feature in the sci-fi epic, but they were replaced by George Clooney and Sandra Bullock.

The reasons for Jolie’s opting out were simple: she’s a busy woman, and rather than waiting for the picture to come together, she went straight into Maleficent before directing her second feature, Unbroken. Cuaron provides little insight into any other reasons Jolie could have opted out of the complicated production, but it’s easy to guess: she has a slew of children and the world’s sexiest man to look after back home.

Despite his talent as an actor, Downey Jr. was uncomfortable with the film’s technology.

Meanwhile, Cuarón stated of Downey Jr’s departure to the Hollywood Reporter: “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,” he added. “[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, which was released in 2013, went on to gross over $723 million worldwide, making it a great hit. The decision to switch performers paid off spectacularly, and the studio had to be pleased with how things turned out.

Gravity has been nominated for ten Academy Awards, including Best Film and Best Director, in this year’s Oscars. Cuarón won Best Director at the BAFTA Awards earlier this week, and the picture had six nominations.

Cuaron has no plans for a sequel to Gravity, but anything he does will be very different, as he states, “Once I finish a film, I never see them again.”