tips for a guitarist playing the bass?

yeah when doing bass you have to be much more sensitive to rhythm and dynamics, you're under the guitars so you have to keep it steady so it doesn't sound like a mess popping over and under the rest of the mix. As my man Gary says, feel the groove, that's pretty much the secret. oh and if you're using a pick (and if you're coming from guitars you should stick to a pick, finger plucking is a totally different technique that takes practicing to not make it sound like a fart), pick like a man, no pussy picking!
 
Yep, when using a pick, fucking pick with passion and fire, none of this pussy bullshit. The exception is when the part just calls for a quieter, more dynamic bass part, but otherwise when you're playing with full bore distorted to hell and back guitars, slam the pick down hard into these strings.
 
yeah, and just don't start to palm mute all the parts that are palm muted on the guitars,
bass should only be palm muted as an effect for some parts, but not as a standard technique
like on a guitar, just doens't sound very well, especially on faster parts, it just sounds like a fart.
Don't try to play as fast as you can, if the guitars are playing 16ths at 160 or something like that
and you have to try to get as fast as them, just play 8ths, that just sounds better and is easier
to play for you.
For softer stuff, where the guitar just plays chords most of the time, try to accentuate the stuff,
it's more about the notes you don't play, or play the ocatve every second time to get a bit more
attention.
 
Yeah I just recorded a bass player with an insane attack and it makes all the difference. If the bass guitar and interface/di is good that's more than half your jobe done. It's a bit tricky to tune for such playing, though. With such attack the tone jumps up quite a bit so you have to compensate for that.
 
Well everyone said to pick like a fucking man and shit, but make sure you stretch your arm and be prepared for it, because when I switched to bass this summer I fucked up my shoulder for 3 months because the strings are so much thicker.
 
Quit your job, move in with mom, don't shower :p

Seriously though I like bassists that do something creative, don't always just follow the guitars exactly. Find some unique lines that bridge the guitars and drums together.
 
Well everyone said to pick like a fucking man and shit, but make sure you stretch your arm and be prepared for it, because when I switched to bass this summer I fucked up my shoulder for 3 months because the strings are so much thicker.

yeah this, it won´t be easy at first when you start picking and fretting those huge strings

Quit your job, move in with mom, don't shower :p

Seriously though I like bassists that do something creative, don't always just follow the guitars exactly. Find some unique lines that bridge the guitars and drums together.

Of course you should always be creative, but be careful with this. You should try not to get away from the rhythmic part of bass playing, your melodies should be tight on the rhythm all the time, don´t just start doing "lead bass" cause most of the time that isn´t helping the song, just helping you show off. Normally doing fifths, octaves and simple scale runs between notes (approach notes is where it´s at) help to make the song interesting, keep you from getting bored and also makes you more noticed in the mix.

Oh and if you really want to get deeper into the theory part of actually playing bass and not just playing a guitar with thicker strings, I highly recommend this site http://www.studybass.com
 
As has been said, pick hard and consistently, and I'll add a) when tuning, pick hard so you know you won't go sharp, and b) try to pick closer to the bridge

b) depends on the bass imo, you can also find a sweet spot on the bass to pick ;)
but yeah, closer to the bridge than the neck definitly.

the rest has been said allready
 
all of the physical aspects of what everyone said are spot on ... pick hard, pick consistent, avoid palm mutes, don't be afraid to cut your notes to 1/2 the guitars in drill picking etc ...

although Dan seems to think he's Conan or something :D "yeah this, it won´t be easy at first when you start picking and fretting those huge strings"

as for the creative aspect .. when you say "fill in on bass" are you just gonna be learning and playing the previous bass players parts or are they asking you to create new shit?
 
How long you hold a note for can make all the difference on bass. In speed metal it may not apply but in groovy sections it is very important. Sounds like a stupid thing to say but sometimes the bass is just this huge energy that isn't percieved as a melodic, and how you want that energy to compliment the arrangement can be important. If you don't hold notes for the right length of time your mix can turn into swiss cheese, and if you don't have good monitoring when tracking bass this can easily be overlooked
 
To me it falls to this: pick closer to neck, follow drum bass and empower it (and take advantage of it's power too). Then if you're brave enough and ready to fuck up, be original :D