She is so much older.
Her features are softer, her flirtatiousness abated. All that sass and fire kept neatly behind her professorial title. And she lets him lead. She lets his younger self swoop in and out of deductions and only guides him when he can’t seem to see through his emotions. There’s no teasing or presentation, there’s just her hand to hold. A hand that no longer keeps a sonic blaster but his sonic screwdriver. This isn’t the wild River that baits Sontarans, she’s the Professor that slips her husband a note with a kiss before going to work. She’s so familiar and intimate and that’s why this moment kills me, because she’s giving it all up. She knows that whatever happens today will write their entire story - and without even blinking, she still lets him choose.