Hey, hey, hey...I've got an idea. I don't particularly feel like participating in the game proper, but I feel like commenting. So, I might use the Statler and Waldorf method, and invent two characters to heckle from the sides. However, they cannot be seen or heard by the main characters, so the heckles can't actually be heard. Well, maybe not heckling as much as MSTing...
Jack 'Jimmy' Glue: Not an Assassin, but just a plain assassin, he was ordered to murder the head of the Assassin's Guild during the Ankh-Morpork Civil War by Lorenzo the 'Kind'. Being a blunt instrument of the now vanished monarchy of Ankh-Morpork, Jimmy Glue thought that he knew better than the Assassin's Guild. He was wrong. His death was...interesting, especially by Assassin's Guild standards, who make a point of being polite, efficient, and clean normally. The Jimmy Glue-Pot now resides in their Museum.
Why Death never collected him is up for debate. Jimmy Glue had a nasty habit of making post-mortem one-liners, and Death had often cause to witness them. Death may have a sense of humour, but he is also a professional. Jimmy Glue is merely a psychopath. Whatever the cause, Death has refused to collect Jimmy Glue, and Jimmy Glue now haunts the Assassin's Guild. He tends to hang around the office of Alice Band, wishing that he was tangible enough to try and seduce her. That's when he is not being annoyed by the other resident ghost of the Assassin's Guild...
(And yes, he is an exaggerated version of a certain well-known fictional spy.)
Indecisive Ian: This epithet really only applied to him in life. A contemporary of Rincewind's during his (brief) tenure at Unseen University, Indecisive Ian (last name...err...can I get back to you on that? I'm a bit indecisive) is not a wizard, because he never stayed long enough to become an alumnus. He has tried being schooled at every Guild in the city, even the Seamstress' Guild (do not ask). He didn't even stay for longer than a minute at the Fool's Guild, which was just as well. But then, he joined the Assassin's Guild...
Indecisive Ian was out of his depth, but it was the kind of out of his depth that he wanted to strive for. Unfortunately, he made a bad mistake. He accepted a sherry and an almond slice from Lord Downey (who was then not the head of the Guild, Dr Cruces was, but Downey was still administering certain...intelligence tests).
Upon meeting Death, Ian made his final and most decisive decision, one which actually impressed Death, who had studied this indecisive young man. Indecisive Ian would become a poltergeist. This suited his annoying, Rik Mayall-esque personality (think Richie from Bottom, only more intelligent and slightly more experienced in sexual matters). Unfortunately, he is a rather incompetent one. It took him the best part of a couple of decades to learn how to lift a book. It'll be a few decades yet before he can throw something this size.
There's a reason why Ian doesn't reveal his last name to anyone. Whenever he applied for Guild membership, he always used an alias.
There. How do those characters sound? And they never see the murder or murders, only the aftermath. Call it narrative imperative.
To sum up, Jimmy Glue is basically the Sean Connery version of Bond turned into more of a thug, and Indecisive Ian is basically Rik Mayall.