u create a check box(put a value in it, say 1), for each of your column, and put it in the header part, and also create a check box(put a value in it, say 1) for a row, and put it in a row in body part. This way, while filling a cell, check for its row and column check box, whether it is 1 or empty, if it is 1 then perform whatever operation u want, else simply reject the case.
You might want to look into Replace Field Contents. This only fills down and will affect all rows visible in your table, but if you can first perfom a find or use omit record to omit the records you don't want changed, you can then fill a given field of the remaining records with the same value, a serial number or even the results of a calculation that puts a different value in every record. Make a back up copy and read about this tool in FileMaker help first before you try to use it.
A horizontal fill is a much more unusual option. Filling all fields to the right with the same value seems a bit strange to me, but if that's what you want, you'll likely need to use a scripted approach that combines go to field next, with insert field to put the same value in each such field. If this is not what you meant, please respond in more detail as their might be a different approach that does not require a script for this.