The actress admits the pair bonded immediately on the set and she treated him to movie screenings and a John Mayer concert during the shoot.
But when it came to ending the film, she had trouble letting go.
She says, "We had so much fun last summer and I got used to my little sidekick. When we wrapped, I thought, 'Now what am I gonna do?' I wanted to go on another road trip.
"We went to D.C. and took pictures of all the monuments.
And now Zellweger, who is currently dating actor Bradley Cooper, is convinced she's got what it takes to be a good mum: "I turned into a mom somewhere between Baltimore and the White House. It was really hot and sunny so I kept reaching into my purse for sunscreen and globbing it on his face.".