1. Jerry has the best nervous twitch of all, which is when things get really serious he gets really comic. So he's always pulling the carpet out from under you if you start to take yourself too seriously.

a) I guess "nervous twitch" doesn't mean that he is nervous?

b) It is some idiom: "pulling the carpet out from under you"?

2. I chose to deal with being in the band in a very kind of singleminded way and I developed a lifestyle that I thought added up to what I was. I don't want to go back there again, so I enrolled on a twelve step programme........

a) I suppose that "singleminded way" means that he wanted to be in the band but he wanted to behave in his own way and thought that behaving in this way it would reveal his true personality (added up to what I was)?

All the best!
A "nervous twitch" is an involuntary and sudden (usually repeated) movement of a muscle. It is a medical condition that some people are born with. The condition is aggravated when the person is under stress.
"pull the rug out from under someone" is an idiom. It means to suddenly remove support (can be money or emotional support) from someone, so that their plans or ideas fail.
Singleminded means devoted to one way or method to the exclusion of any others. (The "twelve step" reference intensifies "singleminded" as if it were an addiction)

So in other words he's saying:

1. When the situation is really tensed Jerry is the one who acts most emotionally of all of us. He starts to behave in a funny way and tries to thwart our plans if we became too serious.

2. When I joined the band I started to behave in one way only and I thought it was a good way for me.... but probably it wasn't.....