Why didn't Prof Lupin allow Harry Potter to cast a spell on the Dementor?

Question

Prof Lupin didn't allow Harry Potter to cast a spell on the Dementor during the Boggart class in Harry Potter and the Prisoner of Azkaban:

Is there any reason?

Answer 1

I believe this is a mistake on the film's part. In the books Lupin stops the Boggart before it can transform before Harry.

‘Riddikulus!’ bellowed Ron, and the spider’s legs vanished. It rolled over and over; Lavender Brown squealed and ran out of its way and it came to a halt at Harry’s feet. He raised his wand, ready, but –

‘Here!’ shouted Professor Lupin suddenly, hurrying forward.

Crack!

The legless spider had vanished. For a second, everyone looked wildly around to see where it was. Then they saw a silvery white orb hanging in the air in front of Lupin, who said ‘Riddikulus!’ almost lazily.

_{Harry Potter and the Prisoner of Azkaban, Chapter 7, "The Boggart in the Wardrobe"}

The same reasoning is given from Lupin for both the book and the film: he didn't want Voldemort showing up in class and frightening everyone.

‘No,’ Harry lied. He drank a bit of tea and watched the Grindylow brandishing a fist at him. ‘Yes,’ he said suddenly, putting his tea down on Lupin’s desk. ‘You know that day we fought the Boggart?’

‘Yes,’ said Lupin slowly.

‘Why didn’t you let me fight it?’ said Harry abruptly.

Lupin raised his eyebrows.

‘I would have thought that was obvious, Harry,’ he said, sounding surprised. Harry, who had expected Lupin to deny that he’d done any such thing, was taken aback.

‘Why?’ he said again.

‘Well,’ said Lupin, frowning slightly, ‘I assumed that if the Boggart faced you, it would assume the shape of Lord Voldemort.’

Harry stared. Not only was this the last answer he’d expected, but Lupin had said Voldemort’s name. The only person Harry had ever heard say the name aloud (apart from himself) was Professor Dumbledore.

‘Clearly, I was wrong,’ said Lupin, still frowning at Harry. ‘But I didn’t think it a good idea for Lord Voldemort to materialise in the staff room. I imagined that people would panic.’

‘I did think of Voldemort first,’ said Harry honestly. ‘But then I – I remembered those Dementors.’

‘I see,’ said Lupin thoughtfully. ‘Well, well … I’m impressed.’ He smiled slightly at the look of surprise on Harry’s face. ‘That suggests that what you fear most of all is – fear. Very wise, Harry.

_{Harry Potter and the Prisoner of Azkaban, Chapter 8, "Flight of the Fat Lady"}

Harry: Professor, can I ask you something?

Lupin: You want to know why I stopped you facing that boggart, yes? I would've thought it'd be obvious. I assumed it would take the shape of Lord Voldemort.

Harry: I did think of Voldemort at first. But then I remembered that night on the train... and the dementor.

Lupin: I'm very impressed. That suggests what you fear the most is fear itself. This is very wise.

_{Harry Potter and the Prisoner of Azkaban}

The change appears to have been made to make Harry's greatest fear, the Dementors/fear itself, more obvious for the audience at the expense of having the initial scene make a little less sense. One could argue that Lupin was going to stop the Boggart before he saw it was a Dementor when he saw Harry was next, he does look concerned. However, the filming of the scene doesn't really make this clear.