Garner and Ruffalo famously starred together in 2004 romantic comedy 13 Going on 30. The duo recently posted a selfie together on Instagram from the set of The Adam Project.
"It was so instantly comfortable and there was just so much trust there. There's just nothing like having known people for a long time," the actress said on Thursday about working with Ruffalo again.
"It was lovely. I just loved being with Mark again," she continued.
The Adam Project, from director Shawn Levy (Stranger Things), also stars Ryan Reynolds, Zoe Saldana and Catherine Keener. Reynolds stars as a man who travels back in time to receive help from his 13-year-old self.
"We're all used to laughing at Ryan, but just have a box of tissues ready because he's going to tear your heart out and feed it to you," Garner said.
Garner also stars in Netflix's newly released family film Yes Day. Garner and Edgar Ramirez portray parents who agree, for one day, to say yes to anything their kids ask for.
Garner told Fallon that H.E.R. appears in the film and performs alongside Garner and her band.