[spoilering. spoiler spoiler. for seventeen seconds of Avengers. when can I stop doing this?]
Tony is talking. (Context: He's actually trying to convince Banner to take a field trip to Stark Industries, in the heart of NYC, and play on ten whole floors of R&D equipment. The lead-in comment is "it's like Candyland".) This is one smart kid offering toys to another. Tony knows that he likes playing with R&D stuff; Tony is horrifyingly smart; Bruce is horrifyingly smart; therefore Tony figures, Bruce would like to play with R&D stuff. He is presuming that Banner is at least tempted by getting to play with science toys, and he is right. Commonalities establish connection.
Banner declines, pointing out that "the last time I was in New York, I kinda broke... Harlem." (Context: At the end of the 2008 movie, the Hulk fought the Abomination, and the two of them together turned a large chunk of Harlem into many small chunks of Harlem. Bruce may or may not know how much detail Tony has on that, but Tony certainly knows; he's watching rampage footage from the Hulk movies on his office monitors when Coulson interrupts his evening with Pepper.) He might or might not think that Tony is making a symbolic offering that he expects to have declined, but Bruce definitely thinks that even if the Hulk never makes an appearance, the awkwardness would be way too much.
Tony employs tactical idiocy. He is known to ramble and also known to be a horrible combination of persuasive and pushy, so nobody, including Banner, thinks anything of him continuing the sell while he walks around. Tony promises Banner a quiet, stress-free environment.
Context: Banner is really introverted and generally pretty people-shy. He was before the Hulk, and he's worse now. Everyone at SHIELD has been promising him a "stress-free environment", but they're going about it stupidly. They're all terrified of him, and when you're that terrified of something, you really can't not try to keep track of where it is and what it's doing. They want enough warning to run. This is a really special kind of hell for introverts: Knowing full well that no matter where you go, someone will be paying some sort of attention to you all the time. His stress-free place was in the rural slums of India, where he is all but invisible unless he is that very moment being someone's doctor. He expects that Tony, knowing how this superhero thing works, will try to reassure him by promising the very latest in Hulk-detection and -defense devices for the Tower lab, plus nine million people to cater to his every whim while he is science-ing. Bruce would want to shrivel up and goddamn die.
Bruce is also not all that keen on this conversation, so he turns back to his work in an attempt to stop encouraging Tony.
Tony jabs him in the ribs with the whatever that is. Banner jumps and goes ow. He does flash tense for an instant, although not for reasons of Hulk. Banner has a good idea of what his limits are and definitely knows that the threshold is not that low. If it were, he would have simply said no to Natasha, and there's literally nothing Nick Fury, or anyone else, could have done to drag him in against his will. What he's actually dreading is going through the long, drawn-out, awkward process of trying to coax everyone else in that wing of the carrier out from under their goddamn desks. It doesn't even matter whether he Hulks out or not; if anything happens that makes them think he might, that still makes him the center of attention, which is just about his least favorite place to be. It's tedious, and repetitive, and kind of embarrassing, and puts him on the spot in a way he hates.
But Tony has not moved. This is unusual. Stark might be a big ol' grab bag of flashy but poorly thought-out ideas much of the time, but even he could come up with the idea of jabbing and running, or even something as sophisticated as throwing one of the lab toys at Banner, if he thought there was any real hazard involved. Getting stuck with a tiny cattle prod by the genius billionaire playboy philanthropist trying to help him find a glowing alien tesseract is, taking into account the rest of Banner's life, strikingly normal. This is the first bit of normal he's run into since he got here. He starts to smile.
Stark leans in, scrutinizing him closely, as if Bruce is perhaps cleverly hiding the Hulk in the frames of his eyeglasses. This is patently stupid. This is also basically the same thing that everyone else has been doing to him since they met him, except other people are doing it at arm's length, and only after having located the nearest emergency exit. Stark is doing it very obviously for effect, and without looking even vaguely scared. "Dear Bruce: Our coworkers are neurotic and retarded. XOXO, Tony."
Cap interrupts with, "Hey!" Cap is about as neurotic as they come, about Hulk things; he is not amused that Tony is fucking around on SHIELD time, he is not amused that Tony is distracting their other superscientist; and he is really not amused that Tony seems to be trying to annoy the guy that smashes entire buildings when he gets mad.
"Nothing?" says Tony, quizzically. This reminds Banner that yes, nothing has happened; he has not actually smished anyone on the Helicarrier, no matter how much they deserve it. This mocks everyone who has been eyeballing Banner warily since he got here. Not only do they not trust Banner to control the Hulk, they also trust neither themselves nor Banner to see the signs of an imminent Hulk-out before it happens, and are so paranoid that they might as well just go around asking him, "Hulk now? No? Now?" until he gets so aggravated that there is indeed HULK NOW. This also suggests to Banner that... Tony does actually kind of want to know what makes it happen, because it's a thing and it's interesting, and a zappy-pen isn't going to do it but surely Bruce have some idea of what the triggers are, and whether it's binary thing or the Hulk wakes up just a little when prodded but not enough to make an appearance. There is also the fact that Tony is ignoring Cap here, which isn't unusual, but the fact that he doesn't even seem to care enough to defend against the implication that he's doing something dangerous suggests that Stark, in fact, does not think it's dangerous at all.
"Mostly harmless" is the kind of sign he hasn't gotten to wear for a while. Banner drops his shoulders and smiles at Stark briefly, before returning to his work. His posture is noticeably less tense now. Cap is yelling, but eh -- Cap does that kind of a lot.
Humans tend to reciprocate trust. Con men use this a lot -- they contrive to put the mark in a position where it looks like the con is trusting them with something, then use that leverage to get the mark to trust them instead. Gestures of genuine trust is how people build relationships. Pretty much all of SHIELD is very respectful towards Banner, but they trust him only slightly farther than they can comfortably spit him. Tony is happy enough to be cooped up in a lab with him, and does the same stupid shit to Banner as he does to everyone else. Except more, because Banner is interesting, and Tony wants to see what kind of interesting things are in his brain. I imagine it's been a rather long time since anyone who knew about the Hulk actually trusted Banner with anything. That's got to be good for something.
The way the scene progresses from here, past where the video cuts off, is that Stark continues to ignore Captain America's attempts to frighten him into not Hulk-baiting. Bruce is sufficiently annoyed by this that he actually puts in a comment about not being there if he couldn't handle pointy objects -- Tony gets another you-are-so-twelve-years-old look here -- before the conversation veers back into the main plot. Tony also decides to wander about, incongruously offering to share his snacks, which is another application of tactical idiocy if ever I saw one. IIRC, Banner shrugs and accepts the snacks, and Cap is too busy being Cap to eat things.
Now scroll back up and realize that I just wrote all that about seventeen seconds of film, and that acting, being artificial in many respects, is typically missing a lot of the very subtle, contradictory, irrelevant, or misleading signals that typify interactions in real life. It's like a never-ending stream of subtitles on multiple readable layers all at once, all stacked up on top of each other. Everyone does this subconsciously, everyday; it's how people work. It's just that transcribing them is bizarrely time-consuming and really weirds people out, for some reason.