Alan Shepard and the other astronauts are going through these ridculous battery of medical tests, including one where they have to go through a barium enema, which involves them in open backed medical gowns crouched over and holding the balloon of barium.
When it comes time for Alan Shepard (who loved to do his own take on the Jimenez character), he has to go from one floor of the building to the other, and finds himself being escorted by a rather large and impressive Hispanic medical assistant. So you see Shepard scurrying around the facility with a tube of barium connected to his ass, crouching underneath a 6 1/2 foot hispanic. They get to the elevator, it opens, and there are several more hispanics in the elevator. So Shepard is basically trapped and helpless in the elevator and he knows what's coming down.
Gonzalez, the large and imposing medical assistant, turns to the crouching Shepard and says, "You know, Mr. Shepard, me and my friends think your Jose Jimenez imitation is A-OK....But what you're doing with it is B-A-D."
And you just see Shepard nodding furiously, knowing how insensitive he had been.
The next shot is out of the elevator, with Shepard walking with Gonzalez with his barium balloon deflated and Shepard walking fully upright again. The exchange goes:
Alan Shepard:
Tell me something, Mr. Gonzalez. You ever have any explosions doing this?
Gonzalez: All the time. It's a mess.
Alan Shepard: Tell me something else, Mr. Gonzalez. How am I doing?
Gonzalez: I think you're going to make it, man. I think you're going to be an astronaut.
Just a classic scene from a classic movie based on a classic book. I love it.
On Edit: Here's a clip of the scene:
http://www.youtube.com/watch?v=acrcGIttN_w&feature=related