When you work in a supermarket or similar outlet, overtime is usually defined as an entire extra shift that you are “voluntarily” working. It is usual practise to get 1 and a half times pay for this time and a lot of kudos from your line manager.
In practise (at Tesco where I am currently employed at least), it is rarely voluntary. Although you can refuse overtime it does not look good and your manager will probably hate you for months afterwards. Taking overtime is not only expected, it is almost mandatory. The usual reason to have to take it is someone calling in sick at a time when your manager knows you will be free. You will then receive a phone call and be guilt tripped into doing it. The problem is, it will always be at some inaccessible time such as 6.00 am on a Saturday.
The other kick in the teeth, at least at Tesco, is that you do not get any extra money, you get the normal hourly rate (for me it’s £3.86), this has the additional meaning that if you do, say, five hours overtime on top of your usually four hour shift (9 hours), you are forced to take an extra hour and a quarter break, (split into at least three), which is unpaid, so, you end up doing nine an extra five hours for only three and three quarter hours pay.
The only real bonus with doing overtime is that after you’ve done it a few times your manager tends to be slightly less stressed with you.