I have seen Star Wars: The Force Awakens many, many times. Every time that I have, there are two scenes that have always caused an audible reaction from the audience. First, the audience cheers, or gasps, when they see the Millennium Falcon the first time. Secondly, the audience breaks out in a fit of laughter every time BB-8 gives his "thumbs up" to Finn via a lighter he has stowed inside his body. It turns out that moment was not part of the original plan, and creating it was actually a lot of work.
According to Neal Scanlan, who was the head of the Star Wars creature shop, the "thumbs up" moment was not in the original shooting script for the scene. While he doesn’t remember any specific details of how the idea came about, he credits director J.J. Abrams with having the idea at some point after the original scene was shot.
I think the reaction that BB-8 has to Finn would have been a practical effect that we shot on the Millennium Falcon, but the [idea of] the thumbs up is something that definitely came later in the day.
Luckily, adding the moment after the fact was what they would have had to do anyway, since BB-8 was designed as a combination of both practical and digital effects. While BB-8, was mostly a physical effect, all of the various tools that he uses throughout the movie, with the exception of the tray that holds the map, had to be added digitally. This meant that when the idea of giving BB-8 a response to Finn came about, they had to digitally create the moment. Roger Guyett of Industrial Light & Magic, explained to MTV what they had to do.
That thumbs up moment is exactly that. I think it’s a real BB-8 and then we inserted a CG panel and its arm that came out. So all of his tools, if you like, when he fires his wires and he does that stuff, that’s sort of digital CG.
Based on this description it sounds like John Boyega wouldn’t even have known that BB-8 was returning his gesture until much later. If it was all added digitally, and wasn’t even conceived of until later, it’s possible he didn’t even see it until much later.
The team apparently had to go through numerous different lighter designs in order to make sure that the gesture was clearly a thumbs up. There was apparently some fear that if it was not done properly it would instead look like the droid was extending a different finger, in a completely different type of gesture. Somewhere there are some hilarious digital effects tests that nobody is ever going to let us see.