Verisimilitude (or, Building a convincing universe on the cheap)

Verisimilitude. That's a big word, ain't it? According to The Almighty Google, the exact meaning is 'the appearance of being true or real' (as in 'Piers Morgan tries for human, but lacks verisimilitude'), and when people talk about realism in books, or movies, or whatever, what they usually mean is verisimilitude. Take Jimmy McGovern's work, for instance; people often talk about his gritty realism, where 'gritty' basically means 'all life is misery and pain', but that has verisimilitude 'cos, in many ways, life is misery and pain.

But how do you achieve that same effect in science fiction, where you may very well be writing about people who aren't human? They might even be sentient five-limbed vampire squids, and when one has no frame of reference for what a sentient five-limbed vampire squid is like in the real world (because none of them return my calls, or even send me a postcard), how does one give them 'the appearance of being true or real'? Well, this is where the 'science' and the 'fiction' get down and dirty with each other...