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The famous understatement was actually made twice by two astronauts. Jack Swigert said, "OK Houston, we've had a problem here." Mission Control said, "This is Houston. Say again, please." Then Jim Lovell said, "Uh, Houston, we've had a problem." On the recording, Swigert is garbled at the beginning, while Lovell is clear, so the recording of Lovell is often heard, leading to the impression he said it, even though Swigert said it first. It's commonly misquoted as "Houston, we've got a problem" or "Houston, we have a problem." Because "we've had" implies the problem has passed, Ron Howard chose to use "we have".
In a commentary track, Marilyn Lovell comments that Tom Hanks exactly portrays Jim Lovell's mannerisms and style of movement.
Bill Paxton, Tom Hanks and Kevin Bacon were all very proud of the fact that they weren't sick on the so-called Vomit Comet - the plane used to simulate zero gravity. The cameramen weren't so lucky.
In some scenes where the Earth can be seen from the windows of Apollo 13, it is one of the photos taken by Jim Lovell and Bill Anders on the Apollo 8 mission.
Jim Lovell wore his old Navy Captain's uniform in the scene where he greets the astronauts aboard the U.S.S. Iwo Jima. When Ron Howard asked Lovell if he'd like to be in the film as the ship's Admiral, Lovell agreed, but pointed out, "I retired as a Captain; a Captain I will be."