Book in question was a Peter Milligan Hellblazer collection featuring (on the page I opened to) Shade the Changing Man.
Guest-Starring is a spell.
Target is self. Duration (not range, duration) is Line-of-Sight. For the duration, the next being looking at you sees the real you as a figure fitting their conception of you, and all subsequent beings laying eyes on you see this vision (though it is based on the conception of the original observer).
No "Little short for a Stormtrooper," no "You're nae William Wallace," if you spread rumors that your dwarf cleric Stonedar has iron-black eyes and wears a coat of fire and has a sword that can reach all nations, then that's what people see when you reveal yourself.
It's important to note two things here. First, you do have whatever crazy abilities you are assumed to have. If they think you can use magic, boom, you can now. THIS WORKS BOTH WAYS, and if a powerful lich assumes your level 17 magic-user to be some powerless nobody or beneath their notice, THEN YOU WILL BE for as long as he's noticing you. The second is that if your secondary target (the initial observer) has no conception OF you (you're a complete stranger) or cannot identify you AS you (it's not as if he's seen you on TV) then the spell does not have its normal affect. Instead you are molded into the role they assume you possess (like if they think you're one of the mercenaries the king hired, or some idiot burglar).
Now this spell gets pretty interesting when you're using it on someone who knows you, because the better they know you, sure, the fewer crazy things they may believe about you. But they are also privy to every crazy story you've ever told, every amazing thing you've done, every underhanded deed you've committed, and every time you've looked foolish, and every lie and boast you've made, and just by dint of knowing you longer they've formed thousands of small assumptions about you without realizing it. All of these color the change you undergo during the spell, but since many of these are noncompatible notions they tumble over each other and you can become a caricature of yourself, flowing like mercury, getting all Duck Amuck up in this bitch.