The Participle

A participle that immediately follows the word it modifies is punctuated according to essential/nonessential.

...It was my friend sitting on the end of the row that won the prize.

...It was John Jones, sitting on the end of the row, that won the prize. If the participle is down the line in the sentence -- that is, removed -- it generally takes a comma in front of it.
...She sat next to the door, scowling at everyone. ...The report from the auditor, presented here as Exhibit 2, is flawed. If the participle is at the beginning of the sentence, it always takes a comma. ...Smiling, she left the office in a rush. ...Frustrated by the job, he decided to talk with the boss. Happy punctuating! Margie