Colin stars in both of the director's movies, and revealed that the script for his latest role was written before McDonagh's debut film was made.
"Just before we started In Bruges, you said, 'If you think In Bruges is good you should see the other script I wrote, it's much better'," recalled Farrell, laughing.
"He said, 'I'm only doing In Bruges to see if I like making films. And then I'll make the real one'."
Looking slightly sheepish, McDonagh puts his hands up and jokes: "That was all complete honesty.
"But seriously, Seven Psychopaths was always going to be a bigger script."
McDonagh wrote the screenplay for Seven Psychopaths at the same time as In Bruges - but there was never any question as to which would be his feature film directorial debut.
"Seven Psychopaths was too big to get my head around cinematically before I dipped my toe in the water," he explained. "As a first-time director, I decided to start with something that was more about things I knew."
A tale of two hitmen, In Bruges was in some ways a character and relationship study set in one place, "and I felt I knew that territory from my work in theatre", says McDonagh. "Seven Psychopaths was like a puzzle, a gigantic cinematic jigsaw. I don't think I could ever have done this film without having done In Bruges first."
:: Seven Psychopaths is in cinemas now.