Common Man might be the most difficult character to discuss in the entirety of A Man for All Seasons. That's because he exists on two different levels: as "Common Man," the guy who occasionally gives monologues directly to the audience and changes the scene, and as the collection of small roles that he fills throughout the play.
We know Common Man best as Matthew, the snarky servant of Sir Thomas More. Matthew's a tough nut to crack: he seems to either respect or dislike More, depending on the time of day. Mostly, however, he saves his harshest words for More's hypocritical peers. Just check out this sick burn Matthew drops after being milked for information by Chapuys and Cromwell:
STEWARD: [...] What I can tell them's common knowledge! But now they've given money for it and everyone wants value for his money. (1.459)
In this way, Matthew serves as an audience stand-in of sorts, presenting a distanced and sardonic view on a situation that every other character takes deadly seriously (and probably for good reason). After all, what do the common people really care about all the intricacies of these political and religious intrigues? Life's gonna go on pretty much the same for them. Not to mention the fact that no one asked for their opinion on anything in the first place.
That's not the only role Common Man plays, either. We see him as a boatman. We see him as a barkeep. We even see him as a jailer. Still, the most brutal role he has to play is that of More's executioner. That's an interesting choice, right? There are all of the powerful people in the play, and yet it's Common Man who does the dirty deed. Hmm, seems like when it comes right down to it, the one-percenters in this play are more than willing to let (or force?) the 99-percenters to take care of all the dirty work.
Common Man doesn't merely exist as an extra, of course—he's also a wholly distinct character in his own right. In fact, his character is pretty darn metafictional when you think about, existing outside of the narrative itself. After all, how else would you explain the fact that he's able to drop future historical details about Cromwell's and Norfolk's arrests? At one point, he even reads from a book that hasn't been published yet.
So what does this mean? Well, this outsider perspective allows Common Man to exist closer to us than the other characters in the play can, which means that he frequently voices things that we're thinking but can't vocalize. We also see that Common Man's the only one who's in on the joke: he's the only one who understands the full extent of the situation, even more than Thomas More himself does. Common Man's final line in the play embodies this idea nicely:
COMMON MAN: [...] It isn't difficult to keep alive, friends—just don't make trouble—or if you must make trouble make the sort of trouble that's expected. (2.795)
You might not necessarily agree with Common Man's assessment, but his insight is coolly detached and ironic in a way that the other characters in the play can't quite grasp. He might not be the most capital "I" important character in the play, but Common Man shapes our understanding of A Man for All Seasons in more ways than one.