Dreams are generally pretty different to real life and lots of strange things happen and yet most of the time you wont notice. The main reason you don't notice is because your consciousness doesn't seem to operate too many critical thinking processes while you dream, making you very likely not to notice something unrealistic that is occurring. You might think something is a bit odd but you will either explain it away or not even think about it.
In a majority of my dreams I never become lucid but here are some reality checks I perform once I suspect that my current stream of consciousness might be processing 'dream' as opposed to 'real' stimuli.
Wrong things that sometimes occur. You can't do these things in every dream, some dreams seem to be more 'real' than others. Generally if you can do a bunch of these or none at all.
- I can fly
- I can will things to move
- I can walk / float through walls / ceilings.
- I can flick a light switch and the light level doesn't change (sometimes the light level does change, waking life got this wrong)
- I can will things to disappear
- I can will things to appear
- I am walking / driving in a familiar area but roads, street signs are wrong / missing.
My dream construct has a big problems generating proper looking reflections in mirrors. If I 'glance' at a mirror for an instant it will probably look fine, but if I start moving around things start getting interesting:
- My image becomes distorted, does not look like me.
- My image isn't 'mirroring' me properly, if I wave my hand etc, although it generally does a good job at this
- My image distorts and takes on a life of its own, suddenly it is another person.
The dream construct doesn't seem to have any idea that when you fall you should accelerate (I guess it's kinda counting on your inbuilt instinct to not jump off high things), when falling or even descending stairs things are sluggish. Falling is just floating at a moderate pace in a downwards direction. If you try in run down stairs quickly you will often start floating in a forward/downward motion, slowly.
Dream characters are a rather simple bunch. Crowds seem to act as a single entity, moving as a mob and exhibiting the same kinds of behaviour. Most people seem unable to interact with each other or me. Generally there is 1-3 'interactive' people that will change their behaviour based on what you do, talk to you etc. They generally can't interact with each other or even notice each other. Occasionally when the dream construct stuffs up you will get two people talking to you at the same time oblivious to the fact the other is talking. For a majority of other people in a crowd, they wont respond to you talking to them etc, unless you grab them and then some kind of behavioural pattern will kick in, which could involving them speaking gobbledygook, exhibiting a behaviour - eg hostility, happiness, or even flirting with you, but they wont hold a conversation.
People seem to be assigned roles which under normal circumstances you would never notice - if a person is programmed to just walk down a street past you and can't do much else, unless you realise you are dreaming you are unlikely to notice as you wouldn't normally approach a random person on the street and start trying to talk to them. I would say the AI in many modern computer games equals or exceeds my dream construct. A crowd can interact with you as a crowd - ie chase you run away from you, or most likely, ignore you.
Fight Flight Dream:
In saying all that, you could be able to do everything above and still not realise you're dreaming. Exactly how the critical reasoning processes get kicked off is still a mystery to me, but fight flight dreams trigger them regularly.
Fight / Flight is one of my common dream scenarios, common to many people, probably some kind of dream training programme, like a fire alarm drill. Either I am pursuing someone or being pursued, generally either on foot or in a car. Whenever some kind of monster / bad guy shows up I seem to immediately realise I'm dreaming. I haven't had a nightmare since I was a kid because whenever dreams are supposed to get scary I either realise it's a dream or I fly above them or will them to disappear etc without realising I am dreaming. A common situation is chasing / being chased up and down corridors, if I am chasing, gravity not working when I'm descending stairs generally gives the game away. If being chased quite interestingly bad guys often don't come within 1-2 meters of me even if I am motionless - they just kind of sit there looking menacing until you starting moving again.
Trade off between dream quality and lucidity:
I have noticed the more lucid I become, the harder it is to maintain the dream, perhaps there isn't enough brain processing power to go around, either your higher mind creates your environment, or it thinks rationally about things. I find I have to 'ride the wave' and try and stick to the 'dream script' in order to maintain the dream.