Office protocol can make it different for one employee to ask anther for help. While no one likes the shirker who never seems to be able to quite get his own projects finished and turns helplessly to peers for assistance, most will willingly volunteer to lend a hand to someone who has helped him or her. (来源：http://www.EnglishCN.com)
If you know a coworker is working through lunch to collate a large client packet, your volunteering to stay and help will be gratefully received and most often returned when it's you who is stuck. I say voluntarily because your offer is not to add up paid overtime hours. It is to help a peer in need.
If your offer is accepted, you do not, however, store it away in your mental favor bank or ever remind everyone what a good person you were for helping------you simply hope the favor will be returned when it's you who is overloaded.