Setting of the Font style in Excel worksheet - total ambiguity !!!

Hello,

After 2 days of experience with Excel sheet managing from C#, I still didn't understand the major feature - how to manage cell ranges.
For example in my actual code I want to set the font style in the cell "A1" to bold:

myExcelWorksheet.Range["A1"].Style.Font.Bold = true;

The result is completely different from what I want - all the cells "filled" after this instruction are in bold, but "A1" remains in regular font.

Where I've been mistaken ?

Thanks in advance.

Pavel.