Juror #12 is kind of a strange bird. One of the first things we learn about him is that he works in the advertising industry; as he says to another juror, "You know, in advertising... I told you I worked in an agency? Some pretty strange people work there. Well, not strange, really. I guess it's just they have peculiar ways of expressing themselves."
The funny thing about this comment is that #12 expresses himself in almost the exact same "strange" way he describes here. When he throws out an idea, for example, he says something like, "If nobody else has an idea, I might have a cutie here. Let's throw it out and see if the cat licks it up."
The unfortunate thing about Juror #12 is that his words tend to have little substance to them. But that doesn't mean he's a bad guy. He tries his best to treat the jury room the same way he'd treat a meeting at his ad agency, saying stuff like, "Maybe this is an idea. It seems to me that it's up to us to convince this gentleman that he's wrong and we're right. Maybe if we each took a couple of minutes."
You can see here that, even though he might have good ideas, Juror #12 isn't used to expressing himself in a really forceful way; he's really more of a brainstorming type. Maybe for this reason, he tends to be easily swayed by the opinions of others.