Alex Norton multiple parts

As I see it, the last scene reveals that the underlying concept of the whole film is that everything we've seen has been part of a lanternist's show (as the film's subtitle makes clear -- "A Lanternist's Account of the Tolpuddle Martyrs and What Became of Them"), in which the simple slides and storytelling had been magicked by the audience's imagination into a real-life account. Olivier's film of Henry V (1944) does much the same when the stage and rudimentary props shown in the opening prologue give way to the imagined real-life, location-shot setting of the main body of the film.

All the Alex Norton characters in the film therefore represent the lanternist popping up every now and then to remind us that we are watching a "show" that he is presenting. The fact that many of his characters are, like the lanternist himself, also makers of images -- the diorama showman, the silhouettist, and the photographer -- underlines this association. But it doesn't matter which characters he takes on; ultimately it's just a reminder that he's there behind the scenes pulling the strings and pushing the levers of the drama.

To me, he represents Bill Douglas's alter ego. He's the director of his lantern show as Douglas is of the film.

All of his roles in the film are linked to pre-cinema optical devices, e.g. the Lanternist (magic lanterns, obviously), Sgt Bell (peepshow), McCallum (camera obscura). Bill Douglas was obsessed with these devices, and Comrades is also a film about film - or a film about pre-cinema, to be more exact. There's no significance in the number of roles Alex plays - Bill just put in as many devices as he could.