Bob has a bazooka that shoots laser-blade, 6-feet, twin-edged chainswords on fire.
Downplayed: Bob uses the chainsaw in a fight against a fence post. The chainsaw is effective.
Bob had been suddenly attacked without warning and needed to grab anything he could to use as a weapon. A chainsaw hanging up in his garage was the best thing he could find.
Bob needs a weapon with a high intimidation factor (Probably against a non-zombie opponent), and a chainsaw fits the bill nicely.
Bob is attacked by zombies in a lumber yard, or perhaps a chainsaw factory.
Inverted: A chainsaw is used as a shield.
Bob attempts to use a chainsaw in a fight, but it backfires and he accidentally decapitates himself.
Bob appears to be using a chainsaw as a weapon but this is only a diversion. As soon as his enemies close in for attack he tosses the chainsaw to the side and pulls out a gun.
Double Subverted: Bob does the above, but the enemy knocks the gun out of his hand, forcing him to use the chainsaw.
Parodied: Bob walks into a store to buy a chainsaw. The box with the chainsaw in it states that itís great for cutting down trees, making fire wood, trimming bushes, and killing zombies.
Zig Zagged: Bob puts the chainsaw to great use, but it gets jammed with blood and guts at a critical moment. He cleans it and uses it again, but other characters point out that it's just not worth the trouble, convincing Bob to use a more conventional weapon. At the very end, Bob kills the Big Bad with the chainsaw anyway.
Averted: Upon being surrounded by zombies Bob pulls out a sword to use as a melee weapon.
Enforced: In an attempt to reboot a 1970's children's cartoon as a Darker and Edgier comic series, Lumberjack Bob starts using his chainsaw as a weapon against zombies who are ruining the ecosystem.
"Bob, there are hundreds of different reasons why that is an absurdly poor choice for a weapon."
Invoked: Bob, a huge fan of zombie films, pulls out a chainsaw and proceeds to imitate various moves that he's seen in these films in order to dispatch his undead foes.
Defied: Bob goes to use a chainsaw, but someone points out that itís not a effective weapon and instead instructs him to use a sword.
Discussed: "Zombie outbreak? I'll bet Bob is going straight for the tool shed."
Conversed: "How does Bob carve through fifteen zombies without the blade ever getting stuck once?"
Bob uses a chainsaw but it turns out to be a horrible weapon. Itís too heavy and cumbersome to swing around with any effectiveness. Furthermore Bob could end up cutting himself if he is not careful. The chainsaw can also run out of fuel, leaving Bob without anything to fight with.
The chainsaw jams from too much zombie flesh and explodes in Bob's face
Chainsaws, being too messy, can spread the zombie's blood and contaminants (virus, bacteria, chemicals, etc.) all over Bob, turning him into a zombie as well.