I remember wondering how they depicted weightlessness in the film, because that was a really tricky thing to do before the 2000s. And what you see in the capsule scenes LOOKS real. Turns out, it is.
I finally learned that all the scenes shot in the Apollo capsule and ship cabin were actually shot on a famous airplane that can simulate no gravity in a special way. Many people who have been on it, call it the "Vomit Comet," due to the queasiness some passengers have from the extreme changes in gravity during the highs and lows the pilot takes the plane on.
I was not only amazed, but realized what kind of effort that would have taken. The movie-makers basically would have had to transport entire pieces of the capsule sets, put them inside that plane, as well as bringing the actors and crew, and had to take one or two shots at a time. In fact, they were documented as having to go up over 130+ times to film all the space scenes. Incredible!