The show was originally to be called Our Mutual Friends however the BBC said it would only agree to commission a series if the first word was dropped - so as to avoid any Dickens confusion.
Armstrong and Warren had only briefly met prior to filming Mutual Friends - they'd both played together in the same game of poker.
Armstrong says it was on the third day of filming that they became friends. He said:
I was driving us both in Patrick's E-type for the first time and there was a photographer on set. Without saying a word, the pair of us started pulling terrible stagey poses every time we sensed his lens was on us. God knows what those photographs were like, but during that day we dropped our guards, Marc and I became friends and the Patrick/Martin double act was born.
Source: Radio Times
Mutual Friends was Marc Warren's first main comedy role. The actor has since said he was very nervous about doing it, as he felt very much like he was being assessed by 'the comedy people' in the first few days.
When it was announced that Keeley Hawes had also been cast in the flagship BBC One drama Ashes to Ashes, the Mutual Friends shoot schedule had to be brought forward - this lead to some frantic weeks of read-throughs and re-writes for writers Anil Gupta and Richard Pinto so that the scripts could be ready in time.
Source: Broadcast Interview