I've read any number of novels where the middle section moves the protagonist to a new setting and surrounds him with new characters. One of the difficulties of this sort of structure is that, in a lot of ways, the story sort of starts all over again. The author risks slowing things down to a crawl while she introduces setting, characters, backstories and all sorts of exposition. I cast my mind back to Jhumpa Lahiri's The Namesake, which does this "relocating and starting over" several times during the second act.
My work-in-progress has just moved into the second act, and I've written the first chapter of the middle. My challenges were to introduce a new setting and at least four new characters, as rapidly as possible while still keeping the action moving forward and not letting all the exposition display itself as exposition. Those are the same sorts of challenges we face at the beginning of a story, and I think it's essential that we meet those challenges the same way, with the same ideal of keeping the action going.
Rather than prolix and beazle on in the abstract, I think I'll just show you the first part of Chapter Ten, where my WIP's Second Act starts, and comment on what I'm doing as I go along. This is a pretty long excerpt, so you might want to come back and read all this when you have more time. Just saying. Also, this is still a bit rough since I wrote it only a few days ago, so all the usual caveats. Ready? Here we go:
(My explanatory notes are all in blue italics below.)
A local legend, very ancient, tells of the giantess Hvenhild who gathered up an armload of hills in Zealand, put the acres of earth into her apron and waded over the Sound toward Sweden, ten miles away. When Hvenhild was halfway across the water, one of her apron strings snapped and all her thousands of tons of soil fell into the Sound. I imagine the giantess looking sidelong at the great pile of hills she’d dropped, pushing at it with her toe for a moment and then lumbering off through the waves to sit on the Swedish shore and repair her apron, abandoning her lost acres without another thought. When Hvenhild was long dead and we Danes came to settle the island, we named it after her.
I didn't want to jump right in to a description of the new location, and I didn't want to say "So I got on a boat and went to this island." After about four false starts, I hit on the idea of introducing the island location by telling the myth of its origin. I may end up cutting this paragraph, as it's not really necessary, but I like the image of Hvenhild.
Hven rises up from the waters abruptly, a flat bluff with steep white cliffs all around and ringed with dangerous, rocky beaches. There is one small harbor on the north side of the island, where a steep road angles up through a gap in the cliffs, and visitors to the island are obliged to climb up hundreds of feet to the peasant village of Tuna. Standing, finally, on the rolling plain above the cliffs one can turn and look northward to see Kronberg, seven miles distant. The castle is a brick red and coppery green smudge on the faraway shore. Hven is likewise visible from Kronberg, the cliffs a white line floating on the Sound and the bell tower of the church of St. Ibb is a dark obelisk, a solitary tombstone. Elsinore and Hven seem far apart, different lands or even different worlds.
It was not snowing the morning I crossed from Elsinore Harbor to the landing at Hven’s north shore, but nor was it a pleasant day. A low mass of gray cloud filled the heavens and the Sound was a seething, undulating thing of gray wave upon gray wave. The coastlines of Denmark and Sweden were buried under three feet of snow, as was Hven. The world looked hewn from ice, ivory and granite.
The crossing took two hours in our small boat. Marcellus had assigned two Danish soldiers as my assistants, named Cornelius and Voltemont. They played dice, drank mulled wine and complained of the cold and wet during the voyage. They complained again when there was no one at hand to unload our supplies at Hven’s wharf, and their loud complaints as we dragged our trunks, sacks and boxes up the road from the wharf to the town were doubtless audible as far away as the moon.
So that's setting, travel to the new setting, and introduction of Cornelius and Voltemont, who will provide some comic relief. The typical style of their speech follows:
“There is no inn on this island,” Cornelius said.
“Nor a tavern neither,” Voltemont said.
“We must carry our trunks two miles, over hills buried in snow and then build our own fire at Brahe’s ruin,” Cornelius said.
“There will be no dry wood.”
“There will be no dry bed.”
“There will be nothing but a hole in the ground, and we three freezing in it.”
“You men,” I said. “Have either of you been to Uraniborg since Tycho left it?”
“Nay,” they answered. Neither man had stepped foot on the island in his life.
“Go to the church and borrow a cart and oxen,” I said. “We will light a fire in Tycho’s kitchen by noon, I tell you.”
Voltemont ran to the church while Cornelius and I stood at the edge of town, stamping our feet and rubbing our arms under our fur cloaks. Tuna was a village of a few score houses built from stone and wood with roofs of thatch. There were no people about but smoke rose from the chimney of every house and we smelled pottage and bread cooking.
That's right, more setting. This time it's the village of Tuna. Tuna is small. Also, everyone is warm and snug except our three plucky travelers.
“Voltemont takes his time,” Cornelius said. “Belike he joins the priest for a meal at the fireside. He will forget his friends, who turn to ice outside.”
Cornelius complaining. Get used to that.
“Nay, here he comes.”
St. Ibb’s is a small stone chapel that is centuries old and has a bell tower the height of six men. Voltemont hurried from the church, coming forth in a cloud of steam from a narrow side door. Before the door closed I saw the great bulk of Father Maltar. He was not smiling. I had almost forgotten Father Maltar.
Introduction of Fr. Maltar. "I had almost forgotten Fr. M" is meant to be ominous.
“That ancient priest refuses us,” Voltemont said. “His cart, oxen and driver are not at the beck of every slave from Elsinore, he says.”
What ho! Conflict! Conflict is good.
“Says he?” Cornelius put a hand upon the hilt of his sword. “Well, we ought at least claim right of sanctuary in the chapel and go inside.”
“Aye,” Voltemont said. “It is warm in the church. We have missed matins, but we may be in time for dinner.”
“Enough.” I picked up my cases and walked toward the church. With each step I sank an inch into the snow and I tried to remember Hven during the summer, when the hillsides flowed under deep green carpets of long grass where sheep and cattle grazed, when crops rippled in a gentle breeze while fish schooled in the sixty linked ponds Tycho had dug west of the observatory. I tried to recall the good smell of the earth beneath the maples where I had read Copernicus in the hours after dining. These memories refused me. I had nothing but the air filled with ice and wind, a low gray sky, the noise of the waves all around and deep snow lying over the whole of the island. My ears and nose felt brittle in the cold.
All of this is to show that Horatio (the protagonist/narrator) is familiar with the island, but he thinks of it differently than he now sees it. So Horatio is not a stranger, but he will still be uncomfortable there. Discomfort is conflict, which is good.
“Bring over your packs and trunks,” I called to my assistants. “We will speak to Father Maltar.”
It was dark inside St. Ibb’s, and humid, but it was warm. Father Maltar took up most of a low bench by the stove, a young priest and a boy who I took to be a villager sat beside him on wooden stools. Maltar did not look up even when I dropped my cases into a pew with no little noise. Cornelius and Voltemont set up their own racket a moment later as they dragged two large wooden trunks over the threshold and across the flagstone floor.
Two more new characters: the young priest and the boy!
“We are on a mission for the king,” I said, stepping over to the stove. I stood beside the young priest, stretched out my hands to the fire and tried to catch Maltar’s eye. “We require your assistance.”
“So you’ve returned,” Maltar said. His voice was low and rumbled deep in his chest. He looked at the grate of the stove, as if he spoke to it and not to me. “The great man’s toad hops across the Sound and into my church once again.”
Hey, looks like Maltar and Horatio know each other and don't get along so well. We jump immediately into their conflict. Maltar is characterized almost entirely through his animosity toward Horatio, since that's the single most important role he plays in the drama.
“Brahe is not here, only the will of the king,” I said. “We serve the king. As do you, Father.”
“Brahe is dead.” Maltar groaned out the words.
“Aye, but the king lives, and we do his bidding.”
“Brahe is dead.”
“Indeed, Father. I have been tasked with removing all traces of him from Hven. You should thank me.”
“Were I a younger man, I’d thank you with a beating.”
A very old priest threatens to beat you up. What's that say about you?
“You are thankfully spared that effort, then. We only require cartage to Uraniborg.”
“I had a dream.”
“Did you dream of an ox cart, Father?”
Fr. Maltar and Horatio will continue to snark at each other.
et cetera, end of excerpt
So that's what I've done to begin the middle of my WIP, introduce new characters into the story and set the stage in a new location. The technical bit that I think you should be aware of is this: whatever you do, do it quickly. Don't waste time. Get the most important information onto the page right away, and use it to show character. The dialogue here all serves not only to move the action forward, but to show us who the people are who are speaking. Cornelius and Voltemont are buffoons. Father Maltar is not the protagonist's friend. The young priest shows himself later to be allied with Maltar. The boy will be kind to the protagonist but there's some mystery about him. Get to the important stuff first; there is plenty of time later to round out the characters if you want to.
Also, just like at the beginning of the main story back in Chapter One, it's important to build mystery and suspense. That's why you never will get the story of Horatio's previous time on the island in one neat chunk, and every bit of information you get will lead to more questions. Questions = mystery = suspense = tension and all of that keeps the reader turning pages.
This was a long post, but I didn't know any other way to show (not tell!) my possibly useful advice about the start of Second Acts. Do forgive my going on like this.