(written from a Production point of view)
Josh Drennen originated the role of Thalen in the Star Trek: Enterprise episode "Proving Ground". He reprised the role in "Stratagem", but was replaced by Christopher Goodman for the character's appearance in "Azati Prime".
Drennen previously appeared in a 2001 episode of Roswell directed by veteran Star Trek director Allan Kroeker. Interestingly, Kroeker also directed "Azati Prime", the episode for which Drennen was replaced in the role of Thalen. Star Trek: Deep Space Nine guest star William Sadler was a regular on Roswell. Drennen was also seen in a 2006 episode of the UPN series All of Us with Richard Riehle. His other credits include a 1999 appearance on General Hospital and small roles in the films Red Zone (2001) and Self Medicated (2005).