Jennifer Garner plays Reese Witherspoon 'Happy Birthday' on saxophone

March 23 (UPI) -- Jennifer Garner brought out her saxophone on Reese Witherspoon's birthday.

The 45-year-old actress dressed up in a marching band uniform to wish Witherspoon a happy 42nd birthday Thursday.

Garner shared a video on Instagram of herself performing "Happy Birthday." The star wore a red, white and black outfit with a matching cap and pompom.

"Hey, @reesewitherspoon! I don't know why you're on my mind today..... #happybirthdayfriend," she captioned the clip.

Garner said on Thursday's episode of The Late Late Show with James Corden that she attended band camp when she was young. She shared a NSFW story about a trumpet player who was sent home after a hookup.

"I went to band camp somewhere in the hills of West Virginia," the star told host James Corden.

"We did have a 'One time at band camp' situation," she said, referencing the American Pie quote. "Oftentimes at band camp you'll have your first experience with someone, some kind of juicy experience."

Witherspoon's Cruel Intentions co-star Sarah Michelle Gellar also wished the actress a happy birthday. Gellar shared a throwback photo from the 1999 movie.

"Hmmmm @reesewitherspoon I know it's your birthday, but I think we could try and pass for high school students!?! Just think about it. #happybirthday," she wrote.

Witherspoon thanked her friends and fans in a post on Instagram, saying she was "feeling so much love" on her birthday. The actress started filming Season 2 of the HBO series Big Little Lies this week.