The Dr Horrible's Sing-Along Blog actress appeared in last night's (April 27) episode as a computer hacker.
Day said that she had come close to meeting the show's stars Jensen Ackles and Jared Padalecki several times in the past, while appearing at fan conventions.
"Meeting them in person, I have to tell you, bowled me over because they are so manly," she told Entertainment Weekly.
"It was overwhelming because I'm not around a lot of jocks and guys' guys. It was like walking into a romance novel a little bit. They have this crazy charisma."
Day went on to discuss her character, who gives the episode 'The Girl With the Dungeons and Dragons Tattoo' its title.
"She has all these amazing qualities that I haven't seen put in one character before," she enthused. "When I picked up the script, I was like, 'I can't wait to say all these words!'"
"She has a strength that grounds her that is unique for the characters I've played. I loved being able to find that."
Photo gallery - 'The Girl with the Dungeons and Dragons Tattoo':