How often have you read a scene with several characters in it, but the focus is only on two of them? Makes you wonder why exactly those others are there, doesn’t it?
The last thing you want is to have characters just standing around. Give them something to do!
Each scene has to move the plot forward, and every character, no matter how big or small their role, plays an important part in making that happen. If there’s no purpose for them to be there, then they don’t need to be there.
Countless opportunities abound for what characters can be doing in addition to exchanging dialogue, thereby avoid the dreaded “talking heads” scenario. The important thing is to keep it relevant to the scene, and not just something random.
Maybe it’s providing (or at least contributing to) the conflict for that scene, or showing the latest development of the main character’s arc.
If you’re not sure what the characters should be doing, try to come up with several alternative versions that reach the same conclusion.
You know what the point of the scene is and what kind characters they are, so take the time to figure out the best way all of you can work together to get the best possible result.