Natasha's spent a long time - her whole life, basically - playing a part. The part she played was always changing, but the fact that it wasn't her, that was consistent. After the SHIELD data dump, with all those roles revealed for the fakes they were, she had to come to terms with who she wasn't, figure out who she actually is. What's actually real about Natasha Romanov.
Some of that has been hard - she still can't figure out where she stands with Nick, now. Some has been easy - her friendship with Steve, for one. Clint's always been that way with her, which is a balm on any worries that she might have reinvented herself wrong.
And some of it's just been fun, like the way Bruce gets tongue-tied every time she talks to him. Making a man's head spin has always been a talent of hers, but now that it's for her own reasons instead of someone else's, it's... nice.
The storm cloud that's been brewing above Tony's head? That's fun too - to watch, not to instigate. (Though, now that she thinks about it...)
He's clearly been dying to talk to her about something, and as those furrows in his brow have gotten progressively deeper the more she flirts with Bruce, she's pretty sure she's guessed the topic. It has the potential to be a sore subject - the conversation might even end badly - but Tony's had that storm cloud look going on three weeks now. It's not going to go away unless Natasha addresses it.
So, the next time she makes Bruce's ears go pink, it's just outside their shared lab, with Tony in the far corner clearly eavesdropping. Bruce, flustered and sensing tension in the air, makes his excuses and escapes to another floor of labs.
Tony keeps at his work for a few minutes, twisting holograms around and muttering instructions to JARVIS. Natasha leans in the doorway, watching him get increasingly fidgety.
Eventually, fiddling with a screwdriver he has no currently obvious use for, he turns to her and says, "Here's what I don't get."
Natasha tilts her head to one side. "How I can flirt with him when I used to flirt with you?"
Tony points the sharp end of the screwdriver her way. "Bingo. He's morning, I'm evening. You have to have known that."
Natasha shrugs. "You were a mission. Natalie Rushman was designed to appeal to you."
"And, what, that includes moiety?" A moue of disgust crosses his face. "Is there anything you spy-types won't do?"
That right there is a path this conversation is not going down. Natasha rolls her eyes. "It was never going to go beyond flirting."
Tony goes still. Good, train of thought successfully derailed. "No?"
"Ten minutes with you and it was obvious that any connection I might make would be superficial compared to the one you had with Pepper."
"Really." Tony's eyes go distant for a minute. "Huh."
"Curiosity satisfied now?"
"Hm? Oh, uh, sort of? I guess?"
Natasha gives him the kind of look that tone deserves.
He rolls the screwdriver between his palms for a moment. "So, uh. Bruce... is not a mission."
Natasha was too good at her old job to react to that. "I didn't hear a question there."
"Huh," Tony says, as if she'd answered one anyway. He nods, decided. "Yeah, okay, I'm done for now."
There's a new look in his eyes, speculative and aimed at Natasha, that drags a weight down her throat to rest low in her stomach as she leaves. He can't seriously be thinking... but yes, of course he can. Tony Stark resists the urge, but he's fundamentally a romantic. An absent-minded one, but sincere. And now she's not only provided outside confirmation that his current relationship is solid, but inadvertently introduced the idea of a second half, a potential complete set.
(If there's anything the Avengers-themed floors at Stark Tower told Natasha, it's that Tony loves having a complete set.)
And now, just by thinking about it, he's introduced the idea to her, and it's just... it's unfair. Natasha has literally just started entertaining the idea of a relationship, with all the romantic trappings she's only playacted at before. Jumping straight from that into a fully formed marriage is too much - the whole thing is too much, she never should have - she needs to get out of -
"Natasha?" It's Bruce. She's run straight to him, without even thinking about it. God, what is she doing? "You okay?"
"I'm fine." She plasters on a smile, one of her old standbys, knows it'll look as sincere as any other one she's worn for him. Or, at least, it should. Going by the frown on his face, he's not convinced. "Really, I'm fine, I just... do you ever have days where you want to run away from it all?"
He gives her a look that says have you met me? and she laughs, feeling a little closer to real for a moment. It's... nice.
Maybe that's all it needs to be.