You are all wrong! When Banner and Stark were in the lab, Stark poked Banner with something. Then in the same thing he said something along the lines of "I learned to control Iron Man with this thing (points to light on chest) - you can control the Hulk with something too." banner then says something like, "That won't work for me." tony then says something like, "Yeah you will, you'll see." Then Banner turns into Hulk on the ship but whatever Tony did hasn't kicked in yet (at least not fully). This is also why Tony knew that Banner would show up at the final battle "Let me know when Banner gets here, okay?" to everyone's surprise because he knows that Banner should be feeling the effects of whatever Tony did to him. Then, at the very end, Bruce and Tony ride off together - cuz now they are close friends who understand each other and share scientific acumen as well. That's my take.

When Banner and Stark were in the lab, Stark poked Banner with something. Then in the same scene he said something along the lines of "I learned to control Iron Man with this thing (points to light on chest) - you can control the Hulk with something too." Banner then says something like: "That won't work for me." Tony then says something like, "Yeah you will, you'll see." Then Banner turns into Hulk on the ship, but whatever Tony did hadn't kicked in yet (at least not fully).

This is also why Tony knew that Banner would show up at the final battle , saying "Let me know when Banner gets here, okay?" to everyone's surprise, because he knew that Banner should be feeling the effects of whatever Tony did to him. Then, at the very end, Bruce and Tony ride off together - because now they are close friends who understand each other and share scientific acumen as well.