 Adam Baldwin , who makes his first appearance as recurring character Knowle Rohrer , originally auditioned for the part of John Doggett , losing out to Robert Patrick . However , the crew remembered Baldwin 's audition later when casting " Per Manum " , and asked him to play the role . Jay Acovone , who portrays Duffy Haskel in this episode , returned in the same role in the season 's penultimate episode " Essence " ; and had also previously appeared in the fourth season episode " Demons " . " Per Manum " also featured a guest appearance by Mark Snow as an unnamed doctor . Snow had been the series ' composer since the first season . 
