Ben Miles (Patrick) insisted that his character drink Guinness because it is impossible to fake in the studio.
When Richard Coyle read for the part of Jeff, he spoke in a Welsh accent. The character wasn't written Welsh, but the producers liked it and hired him. It wasn't until well into the second series that they realized that Coyle was not Welsh, just putting it on for the show. When Welsh actor Richard Mylan was cast as Oliver, he had to put off his accent in order to make the character less similar to Jeff.
The main characters are called Steve and Susan, after the writer Steven Moffat and producer Sue Vertue (who are married).
NBC's adaptation of the show was considered such a sure thing that when the second season UK original was released on DVD in the States it featured a sticker reading "The original UK version of the SMASH NBC hit!" Unfortunately the DVD came out the very same week that the US version was canceled.
Most of the stars were the first people to read for their respective roles.