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The most important thing is steady TIME -- and then the chords -- the best rule of thumb is to just keep it simple -- in dixie/hot jazz/ballroom/polka stuff etc, just a root and a fifth (on beats one and three) in each bar will suffice just fine, you can also just stick to roots here and there to break up the monotony. Go with your gut when it comes to groove. You dont have to do anything fancy (and often hard core academic dixie types specifically do not want anything fancy). Play in the octaves where they are most comfortable to you, but still can be "felt" by the band.

Be aware of the forms -- as sometimes tune jockeys will just expect you know to follow a form that is different than what is on the leadsheet etc- if unsure, ask them what the form will be (AAB, ABA etc) and just use your ears and go with it. If something goes wrong, dont panic, and most importantly keep playing and dont lose the time and use your ears, and get back on harmonically as you hear it. Make sure you sit near the piano/banjo/geetar player. Best bet is to sit/stand between them and the drummer.

Depending on what area of the country you are in (and the generation of the bandleader) once in a great while keys will be dictated as "three down" or two up" and the likes -- up is sharps, down is flats (often accompanied by a hand gesture of the number fingers pointed up or down) -- For example, three down would be Eb maj/Cmin and two up would be D maj/B min and so on
(just throwing this in as this caught me by surprise once)

So, if the leader goes "Number 55, two down, last A intro" you would play tune #55 in Bb maj/G min starting in the middle of the tune at the last time thru the A section and then go to the top and play it down.

If you have any questions, feel free to ask the leader before the tune starts. It is normal to discuss the tune for a short sec before hitting it -- and if there is something you are not comfortable with (ie a transposition etc) let the leader know.

Most of all be steady, as that that is what they are looking for, and will be most noticeable if not there.

Good luck and have fun!

Rather than be redundant, I'll amen everything Doc said. If you trust your musicianship, which I think you do if you went ahead and took this gig, just keep your antennae up, jump in the water and enjoy the swim.

Dear John,
This is my first "dear Jon"letter.
I like all the advice you have been given from all these wonderful
professionals.
Still being a new addition to an established group must be daunting.
You need every sensory in put you can get when you reherse and
perform with these guys.If you could position yourself where you could
see the other members of the rythm section-look 'em right in the eye
and get some insight in to thier mood and maybe be forewarned of what
is going to happen in the next second or two.It also helps get a grove
going between you and one or more members of the section.

If you cannot obtain recorded material on your new group,go to see them play as well as tune up to and play along with records of good dixieland
groups like "The Dukes"
Good luck,
A fellow pervayour of low tones

There have been some great posts on this topic! I'm playing a Dixieland church service this Sunday, subbing in a 7-piece band with whom I've never played. I'm sure that some of the tips in this thread will come in handy!

tubajoe: Great post.

everyone else: Read tubajoe's post.

All I can add is that I've learned a boatload of tunes and gotten myself out of many jams by sitting behind the left hand of the keyboard player. Jon, if there's a keyboard on the job, sit to his/her left if the setup permits it, and WATCH THAT PINKY FINGER! When you don't know which note to play, follow the bouncing pinky.

Oh yeah, did I mention that tubajoe put up a great post? Well, he did.