I just got interviewed by a band (didnt play for them, they just asked me what my tastes were and what songs i knew) to play rhythym and while im pretty confident in my abilities i wanted to know if there was andything i should really really crackdown and focus on. I know plenty of chords and can switch between them pretty effortlessly and have a good sense of timing, is there anything else i should worry about or just focus on chord progressions and switching between them on the fly?
be able to pick up on chord changes. they will undoubtedly want to jam (duh), so you will have to be prepaerd to watch them, read them, be a step ahead if you can. be quick on what chord to hit.
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Funk rhythms will help... if you don't know chord inversions and alternative voicing of chords then check those out too. Learn how to make progressions out of double stops, those sound good and are a nice alternative to fuller chords.
It would help to know what kind of band it is. Metal, blues, funk?
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Be prepared to play all sorts of guitar... acoustic, electric, distorted, clean, chords, fingerpicking, rhythmic riffs, all sorts of strumming patterns, some basic lead stuff doesn't hurt to know. If you can bring something else to the table like background vocals, that helps.

And if you're a rhythm guitarist, brush up on syncopated rhythms and things like that, stuff that's really interesting rhythmically. You don't have to do it all the time, it's just good to know.

The best guitarists out there have a way to vary up their style -- do a variety of tones and effects and techniques to keep it interesting. The rhythm guitar can definitely set the pace for the rest of the instruments as far as feel goes.
other than things already mentioned, i think one of the biggest keys to rhythm guitar is dynamics. i mean its great if you can play the progression and riffs, but to play them with exactly the right volume and emphasis can make a song go from good to great. think of a great rock song, and think of how the rhythm is played. depending on the song, parts of it might be balls to the wall where the song is supposed to feel energetic, but then you can barely hear the rhythm guitar in cause its kinda faded out when the song is supposed to be mellow. you gotta vary your pick attack and volume for different parts of the song. this is especially awesome when using a driven tube amp, cause you can get more gain by just strumming harder in the heavy parts of songs. knowing when to rock out and when to lay back is key. and making sure you emphasize the right beats, so make sure you know time signatures pretty well. i mean emphasizing beat two in a 3/4 time sig is just gonna sound wrong, and you gotta know that.

another thing that kinda ties in with the above is tone. for a few songs i play, at first i couldnt figure out why they sounded bad when i was playing them well. tone was the answer. some songs call for the sound you get through your neck pickup, others sound much better on the bridge. for me, i like heavier stuff on the bridge and more open stuff on the neck. but thats not all you can mess with, you also got those tone knobs on your guitar. ive seen people who completly ignore them (tom delonge sig strat anyone?). me, i love them, cause they can give you so much versitility with a simple twist. knowing that your rhythm part might need you to cut the tone down so you dont have as bright a sound can really make your part stand out, maybe even stand out by hiding itself behind a lead instrument. those are just my takes on a couple rhythm guitar ideas, but hopefully you find something there to help you.
practice alot of rythm to make sure you have the endurance especially for those long songs where you're doing the same thing over and over....you might cramp up.
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