Why does Matt take his pants off you ask? Well, because Ricky was supposed to be doing an interview with Matt alongside Jennifer Garner, but Jen had to step out. So Matt, being resilient as he is, decided to move forward with the idea because he had prepared for it all morning. What ensues is an improvisational banter session between Matt and Ricky. The backstory is that the last time Matt interviewed Jennifer Garner she was with Matthew McConaughey, and Zaller decided to remove his shirt to make McConaughey feel more comfortable. Jennifer then instructed Matt to remove his pants. Matt was not able to remove his pants at the time, and subsequently he received criticism from Internet fans. That said, Matt was not to be deterred this time around. Unfortunately, Ricky was the only one who could witness it.
Go see Invention of Lying, the story is brilliant.