I was playing around with one of my favorite Sims, Dani Gadget, and she was sitting next to Christopher Steel. I saw the option 'Make move' and I'd never seen it before. This is what Dani did...
She sort of yawned and reached her arm over and started to cuddle Chris! And they were only acquaintances! I tried it again with another of my Sims and Hunter Cottoneye from Riverview and it worked again! Both Chris and Hunter happily had the girls kiss them as well! So, it's a really good way to get into a relationship... fast!
Completely unrelated - when Dani was about to do the maternity clothes flip (that's what I call it) she just stood there, cross-eyed: