In the original "trolley problem," a train is hurtling down a track and you see that it is going to hit a group of five people; it will certainly kill them all. However, you happen to be standing next to a switch that can divert the train down another track where only a single person would be killed. Would it be right to pull the switch?In a modified version of the trolley problem, a trolley is hurtling down a track towards five people. You are on a bridge under which it will pass, and you can stop it by dropping a heavy weight in front of it. As it happens, there is a very fat man next to you—your only way to stop the trolley is to push him over the bridge and onto the track, killing him to save five. Should you proceed?Do you see any difference between these two cases that might affect your answer?

Here, you will need to answer this question based on your own moral beliefs. The question is one that prompts you to explore your own moral sense, not to find the "right" answer.
On the most basic level, you should point out that there appears to be a simple calculus here of one versus five lives. Thus on the surface, choosing to save the five lives is the logical choice. 
Next, you might talk about your beliefs of personal culpability, asking yourself if your own faith or moral tradition treats allowing death by inaction as less culpable than actively killing someone. In this case, acting makes you a killer but refraining from action leaves you as a bystander. Under your own moral code, there may be some distinction in how you evaluate those two positions.
The next issue you should raise as you develop your own moral stance is the problem of decontextualization. In the first variant of the problem, there is a clear binary choice -- a switch is set to one of two possible positions. In the second problem, our real world experience would suggest that there is no way we could know if the fat man would actually stop the trolley and no way we could assume that he would unresistingly let himself be pushed and land in precisely the right place at the right time. This means that as you discuss the fat man variant, you might critique the possibility of moral judgment without context.