She's not really sure what got into her. They were just supposed to be faking it to look good for their mission. It's just the glimpsed out of the corner of her eye that they'd stolen his mask when they threw him into the room (it'd only get in the way in the dark) and the closet was much smaller than she expected. She'd felt his breath ghosting across her lips, making her tingle as they stood awkwardly and tried to think of ways to make it sound like they were doing things they weren't.
The darkness gave her the bravery to try the solution of 'not pretending', at least a little.
She's still not sure how the kiss ended with her straddling his lap, his hands gripping her hips and hers tugging his hair and head back to make him more accessible.
It was definitely just supposed to be a kiss.
She's really not sure how that became 'making out'.