Diane Kruger & Joshua Jackson's Lovey-Dovey Play Date
06/19/2012 AT 06:00 AM EDT
The actress came out to the Broad Stage in Santa Monica, Calif., to support her man, who performed during The 24 Hour Plays, presented by Montblanc.
"She held back while Josh walked the red carpet," an onlooker tells PEOPLE. "But at the after party, the couple was fully together, while hanging out with Amy Smart."
The typically private pair – the subject of recent engagement rumors – stuck to the back area of the party where they weren't as noticeable, but the source says the twosome were smiling and laughing a lot, adding, "[They seemed] totally in love."
Jackson, who appeared as "a total gentleman," was in a great mood at the event, the source says, adding, "It was really sweet to watch [the couple together]."
Ashley Greene, Brooklyn Decker, Jessica Szohr and Wilmer Valderrama were also among the celebrities who took part in an on-stage performance.
– Jessica Wedemeyer