Base your plot on unsupported assertions

‘I don’t understand,’ I said, a leaden feeling spreading from my stomach and into my limbs. ‘What did I do wrong?’
‘No,’ said Father Eschaton, ‘you do not understand.’ Light from the highest windows of the temple bathed him in gold. ‘When you destroyed The Machine, you upset the delicate balance of good and evil in the world.’
‘But...’ I frowned. ‘But The Machine was evil, wasn’t it? It fed on people’s souls.’ He nodded gravely.
‘It was evil,’ he said. ‘But it was precisely evil enough. Now there is a dangerous imbalance in the forces of the universe.’
‘Hang on,’ I said. ‘Hang on a minute. Surely we’re in favour of good and opposed to evil. I really don’t see what I’ve done wrong here.’ Father Eschaton hesitated for a moment.
‘There is a balance...’ he began.
‘Why?’ I said. He shifted uncomfortably.
‘Why? Why is there a balance? Why not just have everything good and nothing evil? What’s actually wrong with that?’
‘I...’ He licked his lips and squinted. The golden light seemed to be bothering him. ‘The balance is beyond human understanding, beyond the mere...’
‘You don’t know, do you?’ I let the question hang. ‘You were going to send me back into that volcano, to almost certain death, and you’ve absolutely no idea why.’ He shrugged and mumbled something. ‘What?’ I said. ‘Speak up.’
‘Sorry,’ he mumbled. ‘I just thought...’
‘What? You just thought what?’
‘I just thought...’ He poked at the dust near his foot. ‘Just thought it’d be interesting.’

Use... dramatic... ellipses...

(with thanks to... Carolyn)
With a screeching cacophony of mechanical discomfort, the plane dipped unevenly towards the runway and...
...made contact with the tarmac. The rubber on the tyres instantly shredded, the exposed metal sending a shower of sparks directly towards the stricken aircraft’s fuel tanks, which...
...were of course safely sealed. The plane skidded along the runway, hurtling ever closer to the airport’s observation tower...
...which luckily was still half a mile away, this being a sizeable airport. Wide-eyed and soaked in sweat, the pilot gripped the controls in front of him and silently cursed...
...his decision to wear thermal underwear and to reuse the same pair of disposable contact lenses he had worn yesterday. Then, with the inevitability of a volcanic eruption, the plane...
...came to a halt safely and every single one of its unfortunate passengers...

Act as if you, as the author, have no inside information

(with thanks to M.H. Forsyth)
Why Thomas opened that door, we may never know. It could have been something he did instinctively, some remnant of an ancient primal urge. It could have been an impulse from the depths of his brain, a half-remembered childhood memory tugging gently at his conscious mind. It could have been something he had been planning for weeks, knowing long before now that he would open the mysterious door at the first available opportunity. It could be that he was not human at all, with none of the caution and instinct for self-preservation humanity brought, but a sophisticated android with a pre-programmed agenda for door-opening. There really is no way of knowing without some kind of detailed neural scan.
Maybe his name wasn’t even Thomas. After all, guessing someone’s name based on their appearance is often a matter of pure chance. Suffice it to say, there was a human, or humanoid figure of some sort, opening (or appearing to open – such tricks of perception are possible) something that looked very much like a door, but don’t hold me to that.

Allow autonomy to body parts

He reached out a sympathetic hand. Helen looked up with hopeful eyes, then swung an eager arm towards him and allowed her optimistic fingers to grasp his.
‘Yes,’ she said, her voice strong enough to shock her own ears. He grinned, his confident mouth lighting up his forthright face. Her heart, delighted, seemed to laugh within her chest – the chest which was usually so reserved and timid. She looked into his eyes and saw them grinning at her. But was there something else there? A hint of trepidation, perhaps? Behind their apparent delight, what were his eyes really feeling? It was impossible to know what was going on in those eyes’ head. Was their heart truly in it?