The movie barely mentions either of these two critical concepts. Worse, the main source informing Rico's choices, his high school "History and Moral Philosophy" teacher, Col. DuBois, is barely there in the first scenes, and not mentioned again. These ideas are presented in hit-and-miss fashion, as if they are part of the recruit training after enlistment, instead of why recruits choose to enlist.
Moving the important motivation to enlist into the recruit training has another consequence—we do not get a real sense of the conflict between Rico and his father. As a result, his reaction when his home city is attacked is shallow. He's now an orphan, okay, move on. This robs the viewer of one of the most poignant scenes Heinlein has written, when as an officer, Rico is relying on his master sergeant.
Lieutenant Rasczak (Michael Ironside) is given most of the lines that Col. DuBois has in the novel. So we get "Violence has resolved more conflicts than anything else. The contrary opinion that violence doesn't solve anything is merely wishful thinking at its worst," instead of
Anyone who clings to the historically untrue—and thoroughly immoral—doctrine that "violence never settles anything" I would advise to conjure the ghosts of Napoleon Bonaparte and the Duke of Wellington and let them debate it. The ghost of Hitler could referee, and the jury might well be the Dodo, the Great Auk and the Passenger Pigeon. Violence, naked force, has settled more issues in history than has any other factor, and the contrary opinion is wishful thinking at its worst. Breeds that forget this basic truth have always paid for it with their lives and freedom.
If this was the only shortcut, I would not complain. It is not. Sergeant Zim is another crucial character whose best lines from the book are given to Lt. Rasczak, or simply omitted. Rico's relationship with training sergeant Zim is formative for him. Again, the movie simply ignores this part of the story.
In addition, the movie completely omits Juan Rico's choice to go to officer training, and how this perspective changes his assessment of his life and responsibilities. I can see leaving this out to save time (and provide grist for a sequel). I suspect, however, that the movie's creators were simply in a hurry to confront Rico with the alien bugs.