The trouser fitting usually takes place on the same day as the jacket fitting. However, the trousers would normally be tried on first so that the customer can see the trousers on their own and then also see them with the jacket. While the trousers are important, most customers will see the jacket as the most important item.
The trouser cutter will ask the customer to put on the trousers, then check the fit at various points – including the waist, to check whether that is too tight or too loose. The customer will also be asked how comfortable this feels. There is some personal preference here. One customer might like a close, or fixed, waist, particularly if he only has belt loops to secure it, while another might have side adjusters and like a little bit of room, so the adjustors can both let out and take in the waist. If the trousers have brace buttons inside and might be worn with braces, there might also need to be a little room so the braces can do their job in supporting the trouser.
The customer will be asked to sit down, to make sure the trousers remain comfortable in the fork and are not pulling at the knee. Most customers prefer trousers that are as close and straight as possible in the line, while remaining comfortable when sitting. It is also useful to walk around a little, as it is a mobile garment, and to make sure that you are not standing too stiffly when examining the fit in the mirror.
It is important to remember that any garment, including trousers, will feel different when it has been worn for a few weeks. So if you are unsure of any aspect of the fit, take the trousers away and wear them a few times. If anything still feels wrong, then bring them in and they will be altered.