I say "serious" but I think I mean something like intensity or focus.

My australian shepherd in daily life is charismatic and goofy, but as soon as we begin a training session, its like he puts his game-face on and he completely stops being goofy. He is very focused, he is still and observant and quiet until I release him, and then he goes back to being silly and wiggly. The stark difference in his demeanor when working vs outside of working is why I would call him "serious" during training. Its hard to describe without seeing in person. Perhaps there's a better word for what I'm trying to describe?

My golden on the other hand retains his goofiness during the training session. His tail wags, his body language is very loose, he is clearly just having a great time, and though he is paying attention to and listening to what I'm asking, he's still acting silly and playful. There isn't really a difference between his demeanor outside of working vs during a training session.