For dialogue, I'm going to go straight to the end where Jack shows up and professes his love for Nancy. This is a rom-com trope and there are some great examples (see: When Harry Met Sally) which create quite a shadow. So imagine screenwriter Tess Morris knowing the story builds to this moment, everyone is expecting it, there's this shadow of great culminating scenes in cinema history, and YOU have to write THAT monologue. Well, I think what Tess did is great. Here it is, Jack speaking in front of a houseful of strangers:

Ladies and gentlemen, boys and girls, and friends and members of Nancy's family who I've never met before in my life. I met a girl today, the wrong girl, except she turned out to be the right girl. And this girl who took a chance on me in the most bizarre and romantic way possible wanted me to take a chance on her, but I didn't, you know. I didn't. I blew it like the stupid 40-year-old mid-life crisis dickhead that I am. Nancy, I am so bloody glad you pretended to by my date today, because if you hadn't, then I never would've heard any of your amazing sex theories, or watched you trying to get jalapeno out of your teeth, or witnessed your very instinctive firewoman skills. I would never have got to know that muddy beautiful triathlete face or witnessed that awesome, highly competitive, ass as you were about to get another strike. In fact, if you hadn't have pretended to be my date today, my day would've been utterly rubbish. [begins to choke up] And so quite possibly would've been the rest of my life. [crowd ahhhs] Don't make those noises. I'm right on the edge. Nancy, you said I was an emotional jigsaw and that I should look for the blue bits. I think you might be the blue bits, Nancy. So, what do you say? Quid pro quo?

Some very nice callbacks here, the blue bits is wonderful. And what about turning a line from The Silence of the Lambs into the closer for the monologue: 'quid pro quo'? Who would have thought of turning a line from a psychotic serial killer to try to seal the deal with one's beloved? But it speaks to who Nancy is... and Jack knows that... which is why it works. There's terrific dialogue throughout the script and that is essential in a romantic comedy. Man Up holds up quite well in that regard.