The actress/singer admits she was nervous about appearing alongside Barack Obama's wife - but the president's partner soon put her at ease during a fun dancing scene.
Cosgrove says, "I've been doing the show for a long time and I usually don't get nervous, but having the First Lady on the set, I was really nervous.
"(She was a) nice, pretty normal person. (The dance scene) was probably the most fun thing we did. We all just got to let loose and have a good time."
The episode is set to air on 16 January (12).