This is such an interesting thread. Personally, I don't quite experience instant sexual attraction, but close to it. I need to see/hear someone 'in motion' a little bit to provoke my interest. Spending an hour or two 'with' someone (whether that's literally in conversation/activities with them as themselves, or watching someone performing on a stage or screen- I work on-off in theatre) is usually enough to know whether I have a desire for them on a physical level. Sometimes I need a day or two to sort of stew on it, but often I'll know after that first meeting.
What exactly attracts me to someone is a question I wrestle with 24/7. I tend to just throw up my hands and call it 'charisma'. I don't have a type as such; it's all in the way someone just *is*. It's a particular attitude and sense of humour, combined with a particular expressiveness of features.
Sexual attraction itself, for me, feels like being hungry/greedy for a specific food. It's distracting, you keep thinking of the act and you can almost taste/feel it. You want to skip out on whatever you're supposed to be doing and indulge in it regardless of whether it's entirely healthy. There are, uh, physiological arousal reactions which I won't trouble you all with graphic details of, though I will say at its most intense I've felt brief headaches from it! (But I suspect that's just a stress/frustration response I have due to being a naturally stressy person, and not necessarily a common experience.)
Actions I categorise as sexual are overt sex acts, any kind of physical contact in states of undress or intimate settings like a bed, and kissing anywhere other than cheek/nose/forehead. These are the things I feel compulsion to do with people I'm sexually attracted to. I categorise 'safe for work' contact, like kissing, cuddling on the couch or holding hands in the street, as tokens of romantic attraction- and I virtually never feel an urge to do these. If I was going to imagine or do those it would be because my sex partner wanted them on a sensual level, not because they're part of my desires. Sensual attraction for me is moot- I don't want to do sensual things with people I'm not sexually into.
The emotional level is a bit less clear-cut. I need to spend non-sex time with sexual partners as a trust- and bond-maintaining thing, just like you need to spend time and do fun stuff with any close friend. In a fully romantic relationship, I feel like those bonding acts are usually used as an unspoken method of escalating the relationship towards spending more and more time together and eventually life-combining. For me, they're maintenance, not escalation. That's something I just need to be very communicative about with partners.