In a real life situation, I wouldn't take any action apart from informing the authorities. Utilitarianism does teach that 1 life sacrificed for the sake of 5 is good, but:
- I don't want to be tried for murder
- I do not know anything about these people, the classic argument would go that the single person you sacrifice could be a scientist who could go on to develop a cure for cancer and the other five people insignificant with their contributions to society in comparison. Utilitarianism cannot possibly be applied perfectly to all situations, especially not one like this, as we cannot simulate all possible outcomes.
- Actively killing someone as a sacrifice and allowing five people to die while you're in a passive position are no the same thing.
Doubtless, more people would pull the lever than push the fat man. Throwing another human being onto a track would be much more difficult to do than pulling a level to achieve a similar effect. We also don't know whether or not you can successfully throw the man onto the tracks without placing yourself in danger, and you cannot be certain that the man will stop the trolley. Finally, even labeling him a "fat man" is an issue in itself as "fat" carries certain connotations that, say, "large" may not do.