Not only *should* you laugh on the back line, you should see it as an opportunity.
The audience has just discovered the interesting thing — the game, the emotional truth, or whatever you want to call it — that’s going to drive this scene. Now the people *in* the scene should be realizing this too, but it’s harder to do so when you’re on stage in the “Don’t Think” mode.
Since you now have a good handle on what the scene is really about, you can do a solid for your teammates that were on stage when it comes back. Call one of them on and initiate with that crystal clear idea. Both the audience and your teammates will thank you.
Laughing on the Backline
I don’t get why someone would say this to you (maybe I’m missing the context).
Laughter shows you’re enjoying a scene. It’s an expression of happiness. Why would anyone want to suppress that?