romulus, It is a matter of choice but the dash should not be overused. There is no rule that will tell you when to use it--and when not to. Mr Smith, a well known carpenter, has just started to build the house....correct usage Mr Smith--a well known carpenter--has just started to build the house....special emphasis In the above sentence a comma is preferred because the element that you are setting apart is directly related to the rest of the sentence and unless there is a reason for the special emphasis a comma is the first choice. Mr. Smith--yes, the man that had the affair with my wife--has just started to build the house...correct. The dash is used when there is a sharp break in the train of thought or to emphasize an appositive. It can be a matter of style. If the thought is a personal comment or explanation or not related at all to the topic you can also use parentheses. Mr Smith (Not that I really care.) has just started to build the house. -Don't use more than one set of dashes in a sentence. -First consider using the comma. -Don't write a paragraph and load it up with dashes. It would be better to rewrite the sentences than use too many dashes.

actually, a dash is stronger than a comma. stronger than a comma, but weaker than a period. it's closer to a colon in terms of the degree to which is separates ideas. also, when it is used, it is typed like this (--) only once (-) is a hyphen.

You should use the commas if the appositive is a single phrase that only restates/renames the noun. Your sentence is an example of that. However, if the appositive contains list, you need to use dashes to avoid confusion. "Mr Smith -- a well known carpenter, beloved father, and outstanding civic activist -- has just started to build the house. "