Julia Roberts had "absolutely no idea" who her 'Mother's Day' co-star Jack Whitehall was.
Whitehall has a number of scenes with Roberts in the ensemble movie and he admits he jumped at the chance to be in the film simply because of her.
When they did work together, Jack found Julia very "open and friendly", though he had to "educate" her on his career.
He told heat magazine: "She was the main reason I did it. Just the chance to work with her - you have to jump at these opportunities.
"She was really open and friendly. She had one scene in which she had to turn on the tears and she really delivered.
"I'm quite into my rom-coms and Julia Roberts and Jennifer Aniston are like the DeNiro and Pacino of the rom-com world.
"Julia had not seen my stuff. But I educated her on my canon. But she had absolutely no idea about anything I'd done."
Jennifer Aniston also appears in the film but has no scenes with Jack, so he had to prove to her that they were in the same movie when they met by chance.
He recalled: "After I'd done the film I was in Los Angeles with a friend of mine who was quite drunk.
"He was going down the stairs and she was coming up and he tripped and headbutted her.
"So I had to get him out of Jennifer Aniston's way and while I was doing that I said, 'Oh by the way, I'm in a film with you.' And she went, 'Oh yeah, sure you are'.
"So I had to give her proof I was in the film and then we had a little chat and she was nice. It was a stressful way of meeting her but it kind of worked out in the end."