oh i love these questions because it's fun to be prompted about rustifau . im gonna answer overall then when they do the do.

i think rustica definitely at first ! like i think it starts as very casual affection because thats how rustica is. telling faust he looks lovely today, buying him flowers in the color of his eyes, inviting him out, and at first faust is tsun about it, like how he was in stellato card story where at first he was like "i dont get it" but akira found him later doing the music-listening that rustica introduced him to but on his own. and i think part of it comes from faust not believing he deserves kindness and being confused when rustica gives it to him unconditionally, and another part coming from wanting to preserve that facade of being an evil curseworker, which rustica who is very emotionally intelligent can see the loving and kind person underneath. and eventually faust and rustica get to a point where faust says sure when rustica invites him out (this is canon, serrato rustica card story) and admitting straight up that he looks up to rustica (rusticas birthday line 2023)

sexually i think its very important to remember the one mahoraji audio drama between lennox and faust: faust is a party drunk and a stripper of a man. and rustica is a west. so they're at the point where they're drinking together regularly and faust and rustica are good friends. i think there's definitely a point where they get wasted and fuck. but if it's going to be sober i think rustica makes the first move in that too because hes so casually affectionate. gives faust a sweeping bow and a kiss on the hand as thanks for leading him to where they need to go and it's so casual on rustica's part and faust doesnt know how to react because he's been in the woods for 300 years and barely seen another person so to be so casually shown physical affection shocks him. but also hes barely been shown physical affection in the last 300 years so he THINKS about it . and thinks about it . and when rustica sees him next faust is bright red saying his hello. and rustica knows hes hot and he knows other people know hes hot and the next time faust does something for him he ramps up the affection and again and again until hes driving this poor curseworker crazy faust is jacking off to the thought of what else he can do... and rustica's always willing to help out his lovely friend faust in whatever ways he wants him to, faust doesnt have to do it alone and all ~!