How much agency did Quirrell have?

Question

Based on the book, I always assumed Quirrell was more or less fully responsible for his actions (although given the exchange Harry partially overheard in Chapter 15, Voldemort seemed to have some kind of power over him): everything but the briefest mention suggests Quirrell was willingly serving Voldemort and was in control of his actions.

However, the Pottermore entry on Quirrell implies a very different situation:

While Quirrell did not lose his soul, he became completely subjugated by Voldemort, who caused a frightful mutation of Quirrell's body: now Voldemort looked out of the back of Quirrell's head and directed his movements, even forcing him to attempt murder. Quirrell tried to put up feeble resistance on occasion, but Voldemort was far too strong for him.

Here, Quirrell had very little agency after stupidly stumbling across Voldemort. A contradiction with the book's canon? The only thing these two versions seem to have in common is the feeble resistance Quirrell put up on occasion.

Exactly what is going on here? How much control did Quirrell have over his actions?

Answer 1

There were two stages to Quirrell association with Voldemort.

Stage 1: Following Voldemort's order

When Quirrell tried to rob the Sorcerer's Stone from Gringotts, he had fully in control of his body. This can be gauged from the fact that he could touch Harry Potter.

“Professor Quirrell!” said Hagrid. “Harry, Professor Quirrell will be one of your teachers at Hogwarts.”

“P-P-Potter,” stammered Professor Quirrell, grasping Harry’s hand, “c-can’t t-tell you how p- pleased I am to meet you.”

_{^{Harry Potter and the Sorcerer's Stone, Chapter 5, Diagon Alley}}

But after he failed to steal the Sorcerer's Stone from Gringotts, Voldemort took control of Quirrell.

Quirrell shivered suddenly. “He does not forgive mistakes easily. When I failed to steal the stone from Gringotts, he was most displeased. He punished me... decided he would have to keep a closer watch on me...”

_{^{*Harry Potter and the Sorcerer's Stone, Chapter 17, The Man With Two Faces *}}

Stage 2: Carrying Voldemort around in his head and following Voldemort's order

This stage mostly started just before Quirrell came to Hogwarts to teach. It involved him performing various tasks like drinking unicorn's blood, jinxing Harry's broom, letting the troll inside Hogwarts, and attempted robbery of the Sorcerer's Stone.

“See what I have become?” the face said. “Mere shadow and vapor... I have form only when I can share another’s body... but there have always been those willing to let me into their hearts and minds... Unicorn blood has strengthened me, these past weeks... you saw faithful Quirrell drinking it for me in the forest... and once I have the Elixir of Life, I will be able to create a body of my own... Now... why don’t you give me that Stone in your pocket?”

_{^{*Harry Potter and the Sorcerer's Stone, Chapter 17, The Man With Two Faces *}}

It was not like Voldemort was in complete control of Quirrell's body. Quirrell could think on his own and could resist Voldemort a bit. But in the end Voldemort got the tasks done his way.

Quirrell came back out from behind the mirror and stared hungrily into it. “I see the Stone... I’m presenting it to my master... but where is it?”

“But I heard you a few days ago, sobbing — I thought Snape was threatening you...”

For the first time, a spasm of fear flitted across Quirrell’s face.

“Sometimes,” he said, “I find it hard to follow my master’s instructions — he is a great wizard and I am weak —”

“You mean he was there in the classroom with you?” Harry gasped.

_{^{*Harry Potter and the Sorcerer's Stone, Chapter 17, The Man With Two Faces *}}

Answer 2

In contrast to the last point Vishvesh made..

“Let me speak to him… face-to-face…”

“Master, you are not strong enough!”

“I have strength enough… for this…”

It seems Quirrell is in full control of his body. It doesnt appear that Voldemort has any control at all over Quirrell's body. If he did he certainly would have tried to kill Harry himself in the following:

Harry sprang toward the flame door, but Voldemort screamed “SEIZE HIM!” and the next second, Harry felt Quirrell’s hand close on his wrist. At once, a needle-sharp pain seared across Harry’s scar; his head felt as though it was about to split in two; he yelled, struggling with all his might, and to his surprise, Quirrell let go of him. The pain in his head lessened — he looked around wildly to see where Quirrell had gone, and saw him hunched in pain, looking at his fingers — they were blistering before his eyes.

“Seize him! SEIZE HIM!” shrieked Voldemort again, and Quirrell lunged, knocking Harry clean off his feet landing on top of him, both hands around Harry’s neck — Harry’s scar was almost blinding him with pain, yet he could see Quirrell howling in agony.

“Master, I cannot hold him — my hands — my hands!”

And Quirrell, though pinning Harry to the ground with his knees, let go of his neck and stared, bewildered, at his own palms — Harry could see they looked burned, raw, red, and shiny.

“Then kill him, fool, and be done!” screeched Voldemort.