the MacGuffin joke
I've never understood Hitchcock's "joke" about the MacGuffin on a train. It's clear that it's some kind of nonsense diversion but the punchline just doesn't make sense:
“It’s an apparatus for trapping lions in the Scottish Highlands.” The first man says, “But there are no lions in the Scottish Highlands,” and the other one answers, “Well then, that’s no MacGuffin!” So you see that a MacGuffin is actually nothing at all.
The expected punchline would be that proves the MacGuffin works well, ergo no lions.
the Mongoose joke
Quote Investigator found a version of the joke dating back to the 1880s (or earlier) that does make sense (I paraphrase):
A man carries an elaborate animal crate nailed shut with breathing
holes. He says it contains a mongoose to kill the snakes seen by his
friend who suffers from DT (alcohol withdrawal, severe cases killed an
alarming percentage of alcoholics who tried to stop drinking). The
other man exclaims that hallucinatory snakes aren't real, and the man
with the elaborate animal crate says the mongoose isn't real either.
The distinction between the two jokes, in my opinion, is important. By the time of Hitchcock's re-telling the story has become absurdist, the punchline is almost random. It seems to be a round-about way of saying "Mind your own business." The "MacGuffin" is undefined and it's purpose is completely arbitrary.
But in the earlier version, the crate is made up to look dangerous to impress an alcoholic. The ruse is so convincing that it entices a concerned stranger to ask what's inside. This version seems much closer to the idea of a MacGuffin in a movie production context. While there's actually no mongoose, the box itself is a convincing presentation that signals something dangerous. It doesn't matter that the mongoose doesn't exist, it's the container that people see.
By the time of Hitchcock, MacGuffin had become a wider metaphor for the plot hook: the random thing that everyone is after. But the original mongoose joke suggests something practical a propmaster needs to build that is seen on camera – not a mongoose puppet with fur and teeth but an elaborate animal crate: the more nails holding the box shut the more dangerous the animal inside.
Stage magic is actually just props and acting
Substitute the MacGuffin/Mongoose with a diamond necklace. The plot involves a plucky heiress and a suave thief pursued by a wise old detective. It's not really about the necklace, that's what makes it a MacGuffin, but the necklace needs to be somehow uniquely fabulous since it is the pretext for bringing everyone together.
A "bad" MacGuffin will tell-not-show why the necklace is important. It will have an elaborate backstory and a family curse. It will be the largest and most diamond-y necklace with rotating sparkles – sound effects are heard when it is seen on camera. There is a limit to how far this can go before the necklace becomes comical, like a mongoose puppet with extra rows of teeth – it breaks the suspension of disbelief.
A "good" MacGuffin leads straight to character actions: a worried insurance agent is fretting about the heiress's decision to wear the necklace to a charity ball. He hires extra police to watch her, and the necklace is removed from the hotel safe only under armed guard. The charming thief has already sidled up to the heiress and seen the necklace – he's had access but not opportunity. To stay close he also pretends to be concerned and agrees with the old detective that she should be more careful.
The audience isn't scrutinizing the necklace as an object, instead they are convinced by the metaphorical box. They begin to watch for lapses in security in sympathy with all the characters. Everyone is waiting for an opportunity for the theft. When they discuss the necklace they are not saying how big it is and too bad about the curse. They talk about logistics and suspects, and the audacity of such a brazen heist. No one needs to explain why the necklace is important because everyone is already behaving as if it's the most important thing in the world.
As a stage prop, its more effective to bring out a nailed box and have the actors back away from it, than try to convince the audience a mongoose puppet is ferocious. Like a stage magician's slight-of-hand, it's all confidence-acting and misdirection. You don't have to dispel the trick just as you don't have to reveal there's no mongoose in the box.
a MacGuffin is in medias res
The MacGuffin leads to escalating stakes because it is already important to other characters.
A MacGuffin is different from a protagonist's personal goal because its "value" is established before the protagonist encounters it. The audience (often through the protagonist) accepts the MacGuffin's value by osmosis, but they are never given the chance to form their own critical opinion. It is important because it's already important to others.
Sam Spade only wants the Maltese Falcon because it's important to the others (and one of them murdered his partner). The suave thief desires the impossible-to-steal necklace because its an ego challenge (but the heiress and wise detective appeal to his better nature). The invasion plans are fake. The stolen painting is a forgery.
MacGuffins are safe for plot twists because other characters are invested in them. Revealing or breaking a MacGuffin is not an ego-crushing defeat for the protagonist.
Heist and spy plots often have a protagonist who rejects the MacGuffin, but is recruited or coerced into pursuing it by other characters. It doesn't need to be justified or personal. Treating the MacGuffin as a story-in-progress is a shortcut to get a reluctant or accidental hero into the action.