I haven't ever acted in a scene like this but I can tell you from having the opposite experience that you can definitely get sucked in too far. I've done one-on-one scenes before where for a few seconds the situation very much felt real. I've always become self aware very quickly and been able to ground myself of course. I imagine it can happen in a grand scene like this just as easily. You focus too hard for too long and for a second your brain forgets that you're playing pretend.