Janney, who now stars on the comedy "Mom," surprised the White House press corps at the start of the daily briefing by taking the podium in place of President Barack Obama's spokesman Josh Earnest.
"Let's be honest. I'm better at this than he is anyway. Right? Just between us," she said with a broad smile, before explaining why she was in town — to raise awareness about prescription drug abuse and heroin use in the United States.
"Mom" addresses the issue of people in recovery from addiction. "The West Wing," created by Aaron Sorkin, ran on NBC from 1999 to 2006. The award-winning show followed the fictional presidency of Jed Bartlet, played by Martin Sheen.