The two fics that your description makes me think of are the Swapmeet: Metropolis, by Debby Stark, and Swapmeet: Burbank, by Margaret Bridgnell (I hope I got the titles right), both available on the archive. In both those fanfics, one actor switches places with his/her fictional counterpart. Maybe those are what you're looking for.

Kaethel smile

- I'm your partner. I'm your friend.
- Is that what we are?
- Oh, you know what? I don't know what we are. We kiss and then we never talk about it. We nearly die frozen in each other's arms, but we never talk about it, so no, I got no clue what we are.

~ Rick Castle and Kate Beckett ~ Knockout ~