Depends very strongly on the context. But let's assume the following things...

• We're both in a hideout
• We might not be completely alone 'cause his people would be there, but there are rooms that can be locked
• I'm in his quarters, sitting at his desk, eating a few ginger biscuits and helping myself to a cup of warm/hot cocoa
• I don't hear him come in, he suddenly says my name while standing behind me, like that's completely normal

I'd have to pull myself together real hard to not flinch and not choke on my cocoa. He'd bend down close enough to me (in my headcanon) that I can feel his breath. From behind he reaches under my chin and tilts my head back so I have to look at him. His lips real close to mine, my cheeks bright red like anything.

"K-Know the Japanese expression 'nani'? That fits pretty well here," would be my words, completely flustered. His smirk would drive me crazy, make me melt. And then the kiss would happen on his initiative. I'd kiss him back, at least after a few seconds of figuring out what's even going on. 😅