This is a good question and both options have their merits.  If you request the information in an interview it should be done solely to gauge the client’s current financial standing.  This can be a valuable opportunity to make sure your client has their ducks in a row in terms of substantiation to back up the information in the form.  On the other hand we prefer that the client provides a preliminary version of the form in writing once they have looked at their financial situation based upon their backup.  The information included in the latter case is usually more accurate.  In either case the client never provides fully accurate information and the form must be redone to reflect all applicable information in the best light possible for the client.