Juror #1 seems like a soft-spoken person who's willing to assert himself when he needs to. He makes a decent effort at trying to keep the other jurors in order because as Juror #1, he is also the jury's foreman and is responsible for keeping the group under control.
We see Juror #1 trying to establish himself in the first lines of the movie, saying stuff like, "All right, gentleman. Let's take our seats." And when he doesn't get people's attention right away, he turns things up a notch and says, "How about sitting down?" We can tell from his body language that he respects the other men as adults and doesn't want to babysit them. But he also knows he has a job to do.
As the movie unfolds, the other jurors get angrier and louder with each other. But Juror #1 is always ready to step in and say stuff like, "Now, please. I don't want any fights in here." When things really start to go off the rails, he tries to get the meeting back under control with a claim like: "All right. Let's stop the arguing. Who's got something constructive to say?"
It's no surprise then when we find out that Juror #1 is also the coach of a high school football team. He knows how to control a group without seeming overly assertive, and that sort of thing can really come in handy when the people you're trying to control are almost all quick-tempered.