12 year old in everyone's business

I have a 12 year old son who is very extroverted and sees himself as the appointed entertainer at all occasions.  I cannot have a conversation with anyone when he is around without him butting in and offering a few sentences - even when he has no clue what we are talking about.  He likes to have the spotlight on him but it just feels like he needs to understand that every now and then its good to turn it off.  I also have an 11 year old girl who is very shy and finally coming out of her shell...unfortunately her brother is constantly jumping in to explain what "she meant to say" or otherwise speaking for her.
Seeing all the issues out there, I feel like I should not let myself get so worked up, but it is incredibly frustrating to always having your child stepping all over your words - every now and then it would be nice if he just kept his mouth shut (I feel horrible saying that, by the way),  Is this just his personality and I need to learn to get over it?

[Kevin Kennedy, Ph.D.]

To a certain extent you needn't be bothered about this. However, if he is being rude or inconsiderate in the process, then some limit setting is in order. So, as an example, if adults are engaged in a discussion of which he is not a part, and he helps himself into the discussion, he should be invited to leave the discussion. With peers, including his sibling, they can handle him themselves.