A new trick to me when running an open-table game at a local shop recently was that many of the players used a specific hand signal when they explicitly wanted to say something out of character. A little Googling seems to indicate that this practice may have started with LARPing (live action roleplaying). Here's a short reddit thread about it (some years old, but still relevant).
It still seems a bit weird to me, and we don't do anything like this when we play games, but it might work for your group. Maybe try it and see what people think!
Commentary by memnarch (who has not seen the movie)
Heh. I think Jim has both leveled up his roleplaying and his inability to distinguish what his fellow players are saying. Of course, the group could set up a way for them to always demonstrate whether they're in character or not, but where's the fun in that for us?
So it looks a little more obvious here that DJ is both a) not cuffed and b) dealing with a large box or chest of some kind. It does seem likely that DJ did some selling out in the movie. Hmmm. That would explain part of the trouble with the door. Though maybe there's another part to it as well; evil BB wouldn't have been shown watching the group so closely if there wasn't already a plan by DJ right? Ex post facto betrayal perhaps?
Transcript
GM: Finn, Rose, any last words to DJ?
Finn: You murdering bastard!
DJ: Take it easy, Big F. They blow you up today, you blow them up tomorrow. It’s just... business.
Finn: Business...?
Finn: What are you going to do with your ill-gotten gains? Buy some things?
DJ: That’s the plan, yeah. Lots of stuff so the First Order’s—
Finn: Well that’s good for you! Enjoy your life of luxury while the Resistance gets wiped out!
DJ: That’s the thing. You’re not gonna get—
Finn: I said enjoy it, you dirty, low-down scoundrel!
Rose: I think you communicated to Finn in-character that DJ actually has... some sort of plan to save the day.
DJ: Ah, good! That in-character stuff is easier than I thought.
Rose: Well, for some of us, maybe.
DJ: {turning to leave with his crates of money} Ave atque vale.