Cahill took his proven talent for playing unfettered and unspoiled characters to NBC, where a recurring role on "Friends" allowed him to reach millions. He portrayed Tag, an underqualified but irresistible candidate for the job as Rachel (Jennifer Aniston)'s assistant in a story arc that looked at Rachel's relationship problems and showcased Cahill's "aw shucks" adorableness. When Rachel hired Tag over a more qualified female applicant, audiences knew it was wrong, but sympathized with her fascination. When Rachel dumped Tag, audiences knew it was for the best, but felt for the brokenhearted young man. Next up for Cahill was "Felicity", where he would break away from his nice guy image with a turn as a crazed drug dealer who won't let go of Molly (Sarah-Jane Potts), the girlfriend who wants out. The sinister and intense portrayal gave the actor a chance to show his versatility in a three-episode recurring role, and even as audiences wished for his volatile character to stop his destructive antics, it was easy to empathize with his poisonous but truly heartfelt devotion.

The actor returned to his prostitute-portraying roots in 2001 on an episode of "Law & Order: Special Victims Unit", (NBC) playing a fresh-faced male escort who is victimized by his own madam and a couple with a dangerous fetish. That same year he completed the pilot "Glory Days" for The WB. A Kevin Williamson series looking at the early burnout of a hot young mystery writer, "Glory Days" debut on the network in 2002, following Cahill's scribe back to his hometown, where strange goings on are the norm and where he must solve the mystery surrounding the inexplicable murder of his father.

On the big screen, Cahill had a winning turn as real-life 1980 U.S. Olympic hockey team goalie Jim Craig, who has to find his own innder drive to achieve greatness, in the well-crafted Cinderella sports story "Miracle" (2004).