Here are a few that might fit what you're looking for. I thought I had some other ones (I know I've read some more pretty good ones), but I guess I didn't mark them.

Role Play (http://delanach.livejournal.com/15452.html)
Summary: Set at the beginning of shooting for season four, when Jared and Jensen had spent the hiatus apart making movies and there was a new co-star for them to work with when they got back. The tension between the characters they play bleeds over into their personal lives so they let the characters sort it out in their own way.

You Must Be This Tall To Ride (http://veronamay.livejournal.com/788771.html)
Prompt: J2, non-AU, gunporn/character bleed. Jensen notices that Jared has a thing for Dean playing with guns, so he brings the Colt replica home. Bonus for Jensen roleplaying and being all growly and Dean-voice-like and making Jared admit he loves it.

Best Birthday Ever (http://chase-acow.livejournal.com/196964.html)
Jared is in the mood for role playing, and since it's his birthday Jensen goes along with it. Jared's birthday wish? That Dean gets all possessive of his little brother and much nakedness and sex happens

Whee! Thanks! =DD

How about Method Acting (http://parenthetical.livejournal.com/68775.html) by parenthetical?

Jared doesn't think there's anything wrong about him having a bit of a crush on Dean Winchester. Jared/Dean!Jensen

It has two sequels as well, one Sam!Jared/Jensen and one Sam!Jared/Dean!Jensen

I second this one, it's one of my favorites.

Thanks for the second, love. =D

Woot! parenthetical is amazing!! Thank you!

She totally rocks it! *hugs*

Here are a few more character bleed fics, not quite as plotty as you've described, but dealing more with how acting out the roles of Sam and Dean impacts the relationship of Jared and Jensen.

monkiedude/Inconceriously (http://monkiedude.livejournal.com/56135.html)
sometimesophie/To Destroy You and Me (http://sometimesophie.livejournal.com/56477.html)
gekizetsu/The Crying Game (http://eighth-horizon.livejournal.com/46953.html)
lazy_daze/Character Bleed (http://lazy-daze.livejournal.com/629871.html)

eighth_horizon is a master! I'm not familiar with the other ones, but I thank you for the links! =DDD

So, are you sure self recs are okay? Because I don't usually do this, but....

I wrote a story once upon a time. And while it's not exactly what you're looking for (no role-play, no confusion of reality), there is a metric ton of character bleed. It also has the extra added bonus of being beta'd by one of the best people I know. Let me know if you want the link. ;)

(HAI, BB! Fancy seeing you here.)