Despite being members of a spy squad in the action movie, Simon insists the pair's off screen conversations weren't very macho and they spent most of their time bonding over fatherhood.
Revealing what Tom would speak about on set, Simon said: "We talked a lot about fatherhood. Stuff like what nappies you buy, teaching your kid to ride a bike. Ordinary stuff."
The British actor has a two-year-old daughter Matilda with his wife Maureen, while the Hollywood superstar has three children, five-year-old Suri, his daughter with wife Katie Holmes, and Isabella, 19, and 16-year-old Connor, his kids whom he adopted with his ex-spouse Nicole Kidman.
Simon also revealed that when he felt homesick and "low" because he was away from his family, Tom would offer to stay late and go through lines, or simply offer kind words and advice, and they were gestures the 'Paul' star hasn't forgotten.
He added: "He is kind of superhuman. I mean, he's incredibly good-looking. He's a very good actor. He's built. He's 49 and looks 32. He is beyond criticism.
"He's at the top despite the cynical beatings that he receives. It's because he conducts himself properly. If you start believing all that c**p about how important you are, that's when you stop working, because nobody wants to work with an a***hole."