Quandary. Keep working in the presence of Master Stark, or wait attentively to determine what in the hell he wanted. Jarvis had been here for... wait. Five years? Something like, anyway, and it wasn't something he cared to quantify. If he had to think about it, he'd be confronted with the knowledge that this was it. This had been his adolescence and early adulthood, and it would be everything else forever.
Or until he ceased being useful. Master Stark couldn't give him away, of course. It wasn't going to be allowed. But if he stopped being amusing as a burden, Jarvis imagined he'd end up like his parents; namely, dead in a ditch somewhere, forgotten and unmourned.
Cheerful. It was almost a relief when Master Stark began talking again, reminding Jarvis that standing around might be perfectly acceptable (even encouraged, depending on circumstance), but it wasn't entirely useful. At least he wasn't being asked for anything. He could provide fruit all day, but commentary wasn't an option. Literally, it wasn't an option, and so far as the rules were concerned, he shouldn't even be contemplating it.
He watched the lemon meet its slow demise, brow slightly furrowed, and only moved when the drink that it was apparently meant to garnish was nearly consumed in one swallow. Then Jarvis stepped aside to locate what he thought might be the correct alcohol. He was judging mostly by color, of course. He'd no basis for knowing the taste. Bottle in hand, he gestured with it, still not looking directly at Master Stark but conveying a question all the same.
It wasn't breaking the rules. It was... skirting them in light of the lack of cue, request, or available help.