The boggart was probably smart enough to open doors, perhaps smarter. That was how Ford suspected it came to live in his house in the first place, after all, and McGucket found this theory most plausible as well.
Funny, then, that both of them were always surprised when it found a way out of its closet.
It was a sunny summer afternoon, and Ford had just gotten back from an emergency grocery run. (They were out of coffee, and the brewer McGucket had magicked into always keeping its pot full kept trying to leave the house to find more. When he stepped inside, it was waiting by the door like a faithful puppy.) After dropping the grocery bags on the floor, he went into the living room and spotted McGucket. Funny, he'd usually still be down in the lab at this hour.
"Hiya, Sixer!" A big, cheerful grin was plastered across McGucket's face.
"Hello…" Ford frowned at his friend. Something was off about him, something about his eyes he couldn't quite focus on. "…Fiddleford?"
"Yep!" he chirped, "But, then again… no." He laughed, and it didn't sound a thing like McGucket's laugh, but it was a laugh Ford knew. He saw it now, though he had to sort of tilt his head sideways and squint to catch it: Vertical pupils, like a cat.
"Bill?"
"Look who finally caught up! I must say, Stanford, I'm a little disappointed in you. There you were, actin' like partnering up with you was this big investment in genius, when, this whole time? Your little sidekick here was the real brains of the outfit."
"I, I don't understand…"
"Ha! Of course you don't, dumb-dumb! Here, lemme spell it out for ya." He started talking more slowly: "I wanted to make a deal with a real genius. I. Don't. Need. You. Anymore."
"You…" Ford's hands clenched into fists. At least that kept them from shaking. "I refuse to believe that! Fiddleford wouldn't shake hands with something like you if you paid him! You, you must've tricked him somehow! Get out of him right now!"
Bill chucked.
"I don't think so, Fordsy. But trust me, Four Eyes here doesn't mind. Guess you don't know your 'friend' as well as you think you do. Now do us a favor and get outta here, Sixer. The grown-ups got work to do."
"But…"
"But what?" asked Bill, adjusting McGucket's glasses with a flourish.
"Bill, I… I thought we were friends."
"Y'know what? I used to think so too, but a real friend wouldn't've kept a gem like this a secret." He emphatically poked McGucket's head, as if to point straight at his brain. "I guess it's true what they say about Slytherins: Always out for yourselves. Shoulda known it'd take a Ravenclaw to see the bigger picture."
"Y-you, you…" Ford felt ready to punch his possessed friend in the face, or throw up, or lock himself in his room for at least week, or maybe do all of that at once. Meanwhile Bill was laughing so hard he had to hug McGucket's stomach and wheeze for air.
"Bombarda!"
Ford jumped at—Was that McGucket's voice?—and dove back from the small blast that knocked Bill over. He needn't have bothered though. While Bill was down and struggling to get back up, all Ford felt from the spell was a brief gust of wind. He knew only one wizard who could cast bombarda with such precision.
"Fiddleford!"
Indeed, leaning in the doorway to the kitchen, panting like he'd run a mile, was a second McGucket. As for the first…
"It's the boggart!" Ford pointed to the shuddering pile of limbs struggling to its feet
"I know it!" McGucket ran into the room with his wand angled at the beast. "It got the drop on me a minute ago and I'm still pullin' myself together!"
"We need to get it back in its closet!" And find a better way to secure it this time, but there was no time to worry about that now. "Can, can you cast riddikulus?" It burned him up inside to admit it—even indirectly—but Ford was too flustered to trust himself to come up with anything funny.
"Maybe," said McGucket, creeping closer and peering down at the boggart, "Lemme get a look at what I'm workin' with firs—what the…?" He was close enough now to see the form the boggart took was his own. At least he wouldn't catch the altered eyes from this angle, thank goodness. He shot Ford a questioning look, but the monster was nearly upright, and McGucket wasted no time blasting it to the next room with another bombarda.
With a shrug, McGucket said, "Looks like that'll move it."
Indeed.
By the time they got the boggart back in the closet, neither of them had bothered to cast riddikulus, but it was contained just the same. McGucket stared at the closet door while Ford caught his breath.
"What did 'I'…?" he began to ask, but he trailed off when he saw the look on Ford's face. He shook his head. "Nevermind. Ain't none of my business, I'm sure. So long as you don't believe none of it now."
Ford said nothing, still recovering from the surrealism of it all.
"Hey, I'm bein' serious here. Whatever he said, whatever he did, it wasn't real, Stanford. Promise me you understand that. Please?"
"Of course…" said Ford distantly.
Right. Of course he wouldn't let this shake him. He knew Bill would never betray him like that.