I am currently in a band. And will be taking over as vocalist. Plus, I will play bass. I can't figure out how to scream and play bass for the life of me. What exercises could I do to improve upon this? What can I do?
The trick is to yell very loudly while playing bass exactly as you would usually.

I kid, but really you just have to practice. Same as singing while playing the bass or guitar or whatever instrument.
...it was bright as the sun, but with ten times the heat
I had the same Problem, too. I started out as a rythm guitarist and when we were tired of looking for a singer I tried it.

It's very hard at the beginning. I started with simplifying the guitar parts I played while singing so I could concentrate on what I was singing and shouting.

The main exercise what you need to do is playing your instrument automatically. Play your songs over and over with a metronome. When your good enough, try to hold a conversation while playing. Or just hum your lyrics. Try the songs part by part and add more and more singing and after some time it will work out.

It takes very much time and you have to do it with every single song but you will become used to it.
I sing and play at the same time. Not that solid at it yet, but it works, and as i get better, i write things requiring a more and more sophisticated sense of rythmic independence. It's a skill like any other.

And it's surprisingly hard. I'm a pretty decent guitarist and a pretty decent singer. As soon as i started both at the same time, playing was sloppy and my notes were off. You get used to it though. I guess the very beginnings of this kind of thing would be to play typical campfire'ish songs.

Watch people who play string-instruments and sing at the same time such as Josh Homme, Billy Corgan, James Hetfield, and if you're very specific about bass, check out the late, great Peter Steele of Type O Negative.

I've heard from people who do fry-screamy stuff that they find it easier than singing, while playing, no idea why, probably less notes to focus on - in any case, if you just practice, you'll become better at it. But it does take time, unless you innately multitask like a pro, do it every day and bit by bit, it will get more and more solid.

If it doesn't come naturally to you then yes, all that's going to help you is a metric arse tonne of practise.
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