Carrie Underwood will make her acting debut on an upcoming episode of How I Met Your Mother.  Underwood will play Tiffany, an enigmatic pharmaceutical sales rep and love interest for Ted (played by Josh Radnor), in an episode scheduled to air in March. The CBS comedy series celebrates its 100th episode on Jan. 11, when Stacy Keibler will join the show as a guest. That milestone will also mark the beginning of a guest arch for Rachel Bilson, who may or may not be The One.