The way Casey looks at it is that sooner or later, whether he approves or disapproves, and whether it's going to get them killed or not, Chuck and Walker are going to get stupid together. (So yeah, it'll probably get them killed, and then it'll be Casey cleaning up the mess, disposing of the bodies, shrubbing away the bloodstains - and, of course, report to the Big Cheeses what's happened.)
His getting involved in things is only logical. Unlike Chuck, Casey is not an idiot, and unlike Walker, Casey is not a fool. He's grounded, sensible, rational. With him in the middle, he can make sure they're all going to survive this.
“So uh hey, you like to cuddle?” Chuck asks from the right, just when Casey thinks he's fallen asleep. “Because I really hadn't read you as a cuddling sort of guy.”
Casey grunts. It's a little rude, but, well, it's Chuck.
“No offense or anything,” Chuck says.
“He used to sleep with a teddybear,” Walker says, from the left. “And I've heard he's very ticklish.”
This time, Casey's grunt is meant to be rude.
“Really.” Chuck sounds entirely too cheerful, given the time. “Ticklish, huh? Who'd have thought?”
“Go to sleep, Bartowski.”
“I'm not sleepy,” Chuck says, sounding like the twelve-year-old he is at heart. “Sarah, are you sleepy? Because if you're not, I was thinking maybe we could do something fun together.”
“Like what?” Casey grumbles. “Pick flowers together? Braid each other's hair?”
“Not quite what I was thinking of, but thanks, Casey. I'll keep those in mind for some other time.”
(Being in the middle sucks when the two morons on either side of you decide it's time for a tickle fight, Casey discovers.)