When roleplaying, always keep in mind that a plot always stagnates if someone says 'no' to something and does not build on the scene.
The "Yes Let's" game is from drama, but can be very helpful in writing situations as well. The game starts with one person saying "Let's ____!" the other person automatically must say "Yes, let's!" and does whatever the first person suggesting, but--and this is a very important but--they also suggest something else to do that goes along with what the first person says.
How to put it into one's writing:
"I'm bored, let's go to the mall or something." "Yeah, and then we can get shoes." "Of course. Do you want to go for coffee after?" "Sweet, then I can try to hook you up with the barista!"
And so on.
This can also be applied to actions.
He stepped into the room. She noticed him and patted the seat next to her on the couch. Walking over to her, he smiled, sitting down beside her. He offered his name: "I'm Bob."
The important thing is not only to go along with what your writing partner sets up, but also add to it. When writing, it's nice to have something to actually reply to. If your or your partner ONLY says yes and does not suggest something else (or does not add anything additional to reply to) then the scene can also stagnate fairly quickly.
Give your partner something to reply to by adding to what they already have. Repeat that into infinity and you'll always have a moving plot one way or another.
If for some reason your character would not do what your partner suggested (they can't shop for shoes because they have no feet; they don't want to sit next to the girl because they are socially inept), then offer an alternative that still builds the scene, or COMMUNICATE your issues with your writing partner. Telling someone that you need a little bit more to work with, or something different to work with is perfectly acceptable and any communication ooc regarding ic things should be encouraged.