Notice the emotional change in conversation when 'get heavy' is used. Everyone was joking until the discussion got heavy with personal stories.
Pro Tip 2/3
Topic Transition
'Get heavy' implies a topic may shift from casual to more profound matters. Our light talk about the weather got heavy when climate change was mentioned.
Pro Tip 3/3
Intensity Spectrum
'Get heavy' may also refer to a gradual change, not just a sudden one. As time passed, the playful debate slowly got heavy.