wasn't he close to a second yellow 2/3 games ago for arguing with ref? Nevertheless, you don't want an overly passionate or heated player, often at risk of yellows and game suspensions to be team captain. Its sets a bad precedence to others and youngsters especially nowadays when refs give out yellows for slightest insult. Look at the best captains to grace the game, from Bobby Moore, to Beckenbeaur, Baresi, Cannavaro, Puyol, calm demeanors but assertive. Alternatively you have the likes of Effenberg or Roy Keane who got booked and missed the 99 UCL final

I know which I would prefer, given a choice. Not saying Barella would make a bad captain, but he wouldn't be my first choice. Could be he grows into the role over time under the right guidance and matures but with Conte and Vidal around it will be difficult.
you're thinking pep talks and pre-game speeches, i've always felt actions speak louder than words, but words are important too. Not that I think Bastoni would be a better captain than Barella but at just 20 he has shown a great level of maturity and calmness, consistently one of our best players on the pitch, rarely making mistakes, ever-reliable, laser focused regardless of the craziness on or off the pitch, those are qualities of a great captain. The vocal part will come with time.