Are you talking about selecting which fields to display, or which records to display?  Either way, you are likely to want parameter fields to control it.

If you are looking to restrict records, then this is a simple matter of putting the parameter fields in the Select Expert.

If you are looking to modify which fields display, it's a little trickier.  You can put a whole series of parameter fields to select or deselect whether or not various fields are displayed.  You can even put a formula in the Size and Position criteria to move the fields, so there isn't a lot of blank space.  However, this is tedious to build, and generally tedious for the user to navigate as well.

A much better solution is to identify a few common groupings of fields.  E.g., a manager's view, a technician's view, an accountant's view, etc.  Set a parameter field to indicate which view you want.  Then, create a separate details section (and usually page header section, with the column labels) for each view.  This allows you to format everything nicely.  Then, simply suppress all of the detail sections except the one selected in the parameter.

There are also additional ways to do this with a .Net wrapper, but I leave those answers to Brian, if you need them.