Your mortgage broker can very diligently and carefully explain the process. As long as you meet your commitment dates and provide all of the appropriate documentation that is required of you, you should be in good shape.
Great point! You absolutely have a responsibility to maintain your current credit profile (your profile at time of pre-approval). Taking out new credit for cars, furniture, or other items during the home buying process has come back to haunt buyers in the past.
I agree with both Danielle and Richard. Your first call should be to your mortgage broker/loan officer and make sure that he explains the whole process as well as what is expected of you. You also need to make sure that as a buyer that during the process you do not go out and buy late items on credit as well as be ready to provide bank statements and returns. Also you will need about 6 months reserves if your account on top of your down payment for the property you are purchasing.
If you are the buyer, you are responsible for meeting certain deadlines - such as the mortgage commitment date. There will also be various financial documentation you will be expected to provide in a timely manner. I would discuss the entire process and your responsibilities with your current lender including important dates and turnaround times. Many times they can provide you with a list of what is to be expected so you can start gathering the documentation (tax returns, pay stubs, bank statements, etc.).