Josh Radnor and Britney Spears
Francis Specker /Landov/Retna
03/12/2008 at 07:00 PM EDT
No first-day jitters for Britney Spears
. Arriving promptly at 7:30 a.m. Wednesday on the L.A. lot of the CBS sitcom How I Met Your Mother
with her assistant Brett in tow, Spears quickly got to work impressing her costars.
"It pains me to report that on the first day of the shoot, Britney knew her lines better than I knew mine," actor Josh Radnor tells PEOPLE exclusively. "She's been great to work with."
In the episode, which will air March 24, Spears plays a receptionist
at a dermatologist's office, who falls for one of the show's main characters, Ted, played by Radnor. She is expected back on set tomorrow for a second day of shooting.
Spears first met the cast on Monday during a table read. A set source told PEOPLE, "She was a bit nervous, but didn't let it get to her... She really seemed to enjoy herself and feel comfortable."