First merge of two shots through digital backgrounds created the impression of scenes done entirely in one shot.
Video (HD 1080p)
1) Continuous uninterrupted shots sequences done without using virtual cinematography, only CGI backgrounds and live actors.
2) In the opening scene, as Kowalski flies very close to the camera, astronauts holding a movie camera and boom mic appear to be reflected in his helmet visor. This is an in-joke by director Cuaron. The production crew's "reflections" were added with CGI to make it look like the scene was actually filmed in space, and that the post-production team failed to digitally paint-out those reflections.
3) Alfonso Cuaron was inspired by the film Marooned (1969). Marooned was a film also set in Earth orbit, but didn't have the technical advantages of CGI.
4) Shooting long scenes in a zero-g environment was a challenge. Eventually, the team decided to use computer-generated imagery for the spacewalk scenes and automotive robots to move Sandra Bullock for interior space station scenes. This meant that shots and blocking had to be planned well in advance for the robots to be programmed.